Adsorbents
All adsorbent agents have an extremely porous surface area. Like a sponge,
they possess the capacity of soaking up moisture and to store it inside
their internal cavities. Those are set up as layers, pores and channels.
The adsorption agents themselves are water-insoluble. The stored moisture
will be released to the ambient air again, once the adsorption agent is
heated. There are different types of adsorption agents:
SILICA GEL
Among them, silica gel is one of the predominantly used adsorption agents
against moisture with a wide range of applications. Silica gel consists
of irregularly shaped and porous particles and is a form of silicic acid.
The raw materials to a large extent consist of sodium silicate and sulphuric
acid. Those chemicals are exposed to a chemical reaction which takes place
under certain conditions. The result is pure silicon dioxide with a very
high SiO2-content. Silica gel is inert and compatible
with any kind of materials, except for strong alkaline substances and
hydrofluoric acid. Silica gel is odourless, non-toxic, does not release
fumes and does not corrode metal. The very extensive surface in the interior
of the silica gel particles consists of a vast network of microscopic
pores which attract and hold moisture up to 36% of their own weight without
losing its structure. Silica gel is regenerated at 130°C. This process
can be repeated numerous times without considerably affecting the adsorptive
performance of the gel.
MICROPOROUS WHITE SILICA GEL
The high-density gel is used for the maximum extraction of moisture from
the air. The water vapour adsorption capacity is approximately: 15 weight-%
at 20% RH, resp. 25 weight-% at 40% RH.
Bead size 0.2–1mm those types are predominantly used
in the insulation glass sector for the dehydration of air between the
glass panels. They can be further found inside the desiccant cartridges
and capsules. Other microscopic gels with very high degree of purity are
used with fragrance carriers and for the maintenance of pour ability of
chemicals and pharmaceuticals. They find further applications in the cosmetic
industry.
Bead sizes 1–2 or 2–3mm are used inside the filters of
gas masks, on the insulation glass sector and for the dehydration of liquids
and cooling agents., für Trocknung von Flüssigkeiten und Trocknung
von Kältemitteln.
The bead sizes 1–3mm represent a universal grain size
for drying air and gas in smaller sized equipment and for the drying of
cooling agents.
Bead sizes of 2–5mm are equipped with an elevated adsorption
capacity with excellent low pressure drop characteristics for the drying
of gases (ethane, ethylene, butane, chlorine, natural gas, methane, methylene
chloride, sulphur dioxide, nitrogen and hydrogen).
ORANGE SELF-INDICATING GEL
This self-indicating drying agent (white silica gel) is free from any
heavy metals and therefore environmentally compatible. The gel is naturally
orange when active and at a 6 weight-% saturation level. As the gel adsorbs
moisture, the colour changes into white. The total adsorption capacity
is approximately 23 in weight-% at 40% RH. The range of application is
identical with that of white silica gel. The colour change, however, represents
a great advantage since it allows monitoring of the saturation level.
The gel can be regenerated when heated at a temperature of maximum 120°C
until it turns to its original orange colour.
MACROPOROUS WHITE SILICA GEL
This gel has been designed for liquid water adsorption. The gel is used
for the liquid water adsorption from moist gas and air streams. It acts
as an upstream pre-buffer for the protection of other gels with smaller
pores.
MOLECULAR SIEVES
The molecular sieves are synthetic zeolites. They are characterized by
a precisely defined and regular pore diameter and possess a crystalline
structure. They are predominantly used for the maximum extraction of moisture
from the air. Regardless of the relative humidity, they adsorb about 20-22
weight-% of water vapour. The volume of a molecular sieve is very low
at the same time providing excellent adsorption capacities. Molecular
sieves regenerate at temperatures of 300 to 400°C.
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