# Efficient Adsorbent-Desiccant Based on Aluminium Oxide
## 4 Industrial Adsorbents-Desiccants and Their Characteristics
### Modification of Adsorbents Based on Aluminium Oxide with the Cations of Alkaline Metals
It has been noted that as the layer height of the studied adsorbents increases, their dynamic capacity with respect to water vapour and the time of protective action naturally increases. At atmospheric pressure up to 0.3 MPa, the adsorption characteristics of the NaX adsorbent are higher than those of the aluminium oxide adsorbent, and the protective action time of the layers of these adsorbents is comparable. However, at a higher pressure (0.3 \(\div\) 0.6 MPa), the situation changes, and the adsorption capacity and protective action time of the developed Al\({}_{2}\)O\({}_{3}\) adsorbent become higher. Under these conditions, the use of the developed aluminium oxide adsorbent, considering its higher resistance to moisture, lower cost and lower regeneration temperature, is preferable as compared to zeolite-based analogues during dehydration under pressure. Along with the equilibrium characteristics of adsorbents (adsorption capacities), an important role belongs to the adsorption kinetics, which take into account the dynamics of adsorption and diffusion of water vapour in individual granules of the absorbent and the rate of filling the volume of these granules. Since the adsorbent granules used in industrial conditions are usually shaped like cylinders and spheres of various sizes, it is important to study the influence of the grain size on the adsorption kinetics. The experimental results on the kinetics of water vapour adsorption on active aluminium oxide have been described by the authors of [84; 85; 86]. The influence of the granule size on the water vapour absorption kinetics for the fractions of the studied sample of highly efficient aluminium oxide (0.25-0.5 mm and 0.5-1.0 mm and a granule size of 3.7 \(\times\) 6.0 mm) was studied by the authors of [86]. It has been shown that when the fraction size is more than 0.25-0.5 mm, the rate of water vapour adsorption decreases, which is due to the influence of internal diffusion. This happens due to the presence of small mesopores (3 \(\div\) 7 nm) in the aluminium oxide adsorbent. Mathematical simulation of mass transfer rates based on a quasi-homogeneous model has been performed. The mathematical simulation made it possible to obtain a quantitative assessment of parameters such as the absorption rate constants and to obtain the efficient water diffusion coefficient inside the grain, the value of which was 1.2\(\cdot\)10\({}^{-6}\) m\({}^{2}\)/s. The proposed mathematical model describes quite well the experimental data on the kinetics of water vapour adsorption on a highly active aluminium oxide adsorbent and can be used for modelling the absorber. In this way, the data presented in this section indicate the important role of the acid-base sites of the surface of aluminium oxides in their adsorption characteristics. By means of methods such as heat treatment or chemical modification, it is possible to control the variations in the strength and concentration of active surface sites and other characteristics of AO. As a rule, this type of impact changes the specific surface value, pore size distribution, total pore volume and adsorption characteristics of the material. The adsorption capacity of aluminium oxide in relation to water vapour can be increased by introducing up to 3 mass.% of alkaline cations (Na, K), increasing the concentration of strong basic sites on the surface.