Silica gels are the most commonly used desiccants and consist of 99% silicon dioxide (SiO2). As desiccant in adsorbers for the ventilation of hydraulic aggregates, gears, storage tanks, transformers, etc., silica gel with the color indicator orange-green is best suited.
Due to its high porosity, narrow-pored silica gel has a large internal surface. The pore structure is not uniform, the funnel-shaped openings allow the loading of water molecules in several layers . Thus it is preferably used as a drying agent. The silica gel orange-colorless has a weaker color change, which takes place with a later loading. It discolours with a water load of approx. 20-25%.
Due to its extreme hydrophilicity (attraction of polar substances such as water) and simple regeneration by raising the temperature to approx. 150°C, Silica gel is preferably used as a drying agent. It shows excellent absorption capacity and can therefore adsorb large amounts of water from the air. The maximum capacity is approx. 37 % of the dry weight of the bulk material. The silica gel whitehas no color indicator.
Molecular sieves are desiccants with very specific properties. They are synthetically produced alumina silicates. Zeolites are distinguished on the basis of their crystal lattice structure, as they have pores of different sizes.
Activated carbon has a porous structure and a large inner surface, on whose molecules can attach themselves. The surface is essentially non-polar, hydrophobic and has an affinity for organic adsorptives such as fatty acids, alcohols and esters. Due to the hydrophobic surface properties, the adsorption of water vapour at low concentrations is very low – so it is suitable for the treatment of humid gases or air. It is used, among other things, in ventilation dryers for hydraulics, in cooker hoods and in respiratory masks.