Silica gels are the most common desiccants and consist of 99% silica (SiO2). For use in ventilation dryers for ventilation of hydraulic power units, gears, storage tanks, transformers, etc., the silica gel is orange-green due to the high water absorption capacity of up to approx. 40% of the dry mass, a simple regeneration in the furnace, safe handling, and of the good contrast color indicator, the most suitable drying agent.
With a granule size of 2-5 mm spheres, the pressure build-up is very low and the gap volume optimal to ensure a very good contact of the water molecules to the silica gel surface.
With a properly designed adsorber, dew points of up to -40 ° C are achieved. Depending on the selection of the color indicator in the silica gel, the drying efficiency and the maintenance interval of an adsorber can be influenced. It can be selected from very strong drying and short maintenance interval to a weaker air drying and a long maintenance interval.
- Hydraulic units
- Storage tanks
- Barrel & IBC
|Product flyer||Techn. data sheet||Techn. Data sheet
||Safety data sheet||Safety data sheet||Customs information
|Orange - Green||Orange - Colorless||Orange - Green||Orange - Colorless|
Varieties of silcia gels
A distinction is made in fine-pore silica gels and wide-pore varieties. Fine-pore silica gels have a higher water adsorption at low partial pressures. Wide-pore silica gels have a smaller inner surface, but larger pores. Fine-pore silica gels are used much more frequently in the dehumidification because they are hydrophilic due to the larger specific surface area and the greater number of silanol groups than wide-pore.
Material and technical data
- Chemical designation: Amorphous silicon dioxide
- Composition: SiO2
- CAS-Number: SiO2 63231-67-4
|Appearance and shape||Round, solid balls||Round, solid balls|
|Silicon dioxide SiO2||%||99.00||99.80|
|Water adsorption capacity||ml/kg||>370||>70|
In fine-pore, microporous silica gels the centre of gravity of the pore radius distribution is 1-2 nm (10-20 Å).
In wide-pore mesoporous silica gels the pores are 2-20 nm (20-200 Å).
Critical to the use of silica gels as drying agents are the binding energies, and the loading capacity. For the first molecular layer, the binding energy is 5.140 kJ / kmol.
With increasing loading on multi molecular level, this value drops sharply. This results in a slowing of water absorption in a bulk of silica gel.
Combining fine-pore silica gel-water corresponds to a convex isotherme type. The isotherme shows a steep rise. The mono molecular occupancy of the silica gel is complete at about 30%, then takes a multi-layered occupation of adsorbent surface and capillary instead. At an adsorption capacity level of about 35% it occupies as a plateau and rises slightly.
Regeneration of silica gels
In practice, the most common method used to regenerate silica gel is to increase the temperature. In the desorption of water vapor from silica gel, the temperature must be above 100°C. Recommended for regeneration are temperatures between 150°C to 175°C. However, bearing in mind that one is damaged in the silica gel applied color indicator at these temperatures. Therefore, a regeneration temperature is specified from 121°C for silica gel with color indicator and should not exceed 140°C.
Aging of silica gels
If fine-pore, colorless silica gel is regenerated at a temperature of 150°C, the equilibrium concentration decreased after 100 cycles by about 20% and the half value diffusion coefficient by 32%. Thereafter, the equilibrium loading and the diffusion coefficient reaches a limit value, which is up to 500 cycles no longer significantly below.
Adsorption of further substances
Besides water, also other substances can be adsorbed by silica gels. To ensure the optimal choice of the adsorbents the substances and their diffusion coefficient should be known facing silica gel.
|Mixture||Inert gas||Temperature [°C]||Pressure [bar]|
|strongly bound||weaker bound|
|H2O||C6H6||N1 (1 bar)||50.00||1.00|
|C3H6||C2H4, C3H8||-||0-40; 25||1.00|
|C6H6||n-C6H14||Air (1 bar)||70.00||0.00|
|CH3OH||C6H6, H2O||N2 (1 bar)||50.00||1.00|
|Mixture (decreased binding forces)||Inert gas||Temperature [°C]||Pressure [bar]|
|C3H6 / C3H8 / C2H4||-||25.00||1.00|
|CH3OH / H2O / C6H6||N2 (1 bar)||50.00||1.00|
For visual display of loading with steam silica gel is dyed with an indicator.
Frequently pH indicators are used. In the adsorption of water on the surface of the silica gel, the pH value changes.
This change causes a change in color of the indicator. Further, iron and the heavy metal salts of cobalt (II) chloride are used.
|Color indicator||Color indicator||Hesitation|
|Orange -> Green||pH-indicator||harmless|
|Orange -> Colorless||pH-indicator||critical|
|Blue -> Pink||Heavy metal||critical|
|Orange -> Red||pH-indicator||harmless|
|Blue -> Red||pH-indicator||harmless|
The size of the grain has a strong influence on the adsorption. The larger the particle diameter is...
- the longer adsorption times in the perfused adsorber
- the smaller the pressure loss when flowing through the adsorbents bedding
- the greater the void fraction of adsorbents bedding
- the greater the effect of the wall effect in the adsorbents
- the greater the hydraulic diameter of the adsorbents bedding
- the more uneven the radial velocity profile in the adsorber
For perfused bulks od silica gel balls are recommended with a diameter of 3-5 mm.