People refer to Desiccant Cabinets, and Dry Cabinets, and Desiccant Dry Boxes…but not everyone is familiar with the actual desiccant technology that we employ. Unlike clay or silica, we use a crystal known as Zeolite. It is a molecular sieve — that is to say, the size and shape of its structural openings are that of H2O molecules. And those water molecules are literally sifted from the air inside the cabinet. The desiccant is never touched by operators, and it never needs replacing.
Read on for our more technical explanation:
What is Zeolite Technology?
Zeolite molecular sieves are crystalline, highly porous materials, which belong to the class of alumino silicates.
These crystals are characterized by a three-dimensional pore system, with pores of precisely defined diameter. This diameter is in the dimension of the size of molecules such as water, CO2 and H2S. The pores can be adjusted to precisely determined uniform openings allowing for molecules smaller than its pore diameter to be adsorbed whilst excluding larger molecules, hence the name “molecular sieve”. The different pore sizes of synthetic zeolites open up a wide range of possibilities in terms of “sieving” molecules of different size or shape from gases and liquids.
Zeolite molecular sieves are purely synthetic materials and are characterized by the following properties:
–Selective adsorption due to the uniform pore size of the zeolite structure.
–High adsorption capacity for polar substances at low concentrations
Molecular sieves are manufactured by crystallisation from aluminium hydroxide, sodium hydroxide and waterglass. Under carefully controlled conditions, the crystallisation process produces the required sodium alumino silicate structure. The formed zeolite crystals can then be ion exchanged to adjust the pore size. After drying, the molecular sieve crystals can either be processed to activated zeolite powder or a quantity of binder is added, forming the material into beads. These beads are dried, calcined and finally screened to the required particle size prior to packaging.
In addition, the pore size plays a significant role, allowing or prohibiting the entrance of molecules to the pore system.
Selective Adsorption of Water and other Polar Substances
The up-take of water or other materials in zeolites is called adsorption and functions on the basis of physisorption. The main driving force for adsorption is the highly polar surface within the pores. This unique characteristic distinguishes zeolites from other commercially available adsorbents, enabling an extremely high adsorption capacity for water and other polar components even at very low concentrations.
The adsorption on molecular sieves is therefore dependent on the following physical molecular properties:
Size and Shape: Molecules larger than the pore opening of the molecular sieve cannot be adsorbed, smaller molecules can.
Molecular Polarity: Molecules with large polarity or polarisability can be adsorbed preferentially under identical conditions
Of note is the high capacity of the Zeolite used by Totech even at low water concentration, allowing to dry to very low water contents. The molecular sieve can also retain its high capacity at high temperature, which makes it the optimal material if drying needs to be carried out at elevated temperatures.
The adsorption process is fully reversible and of purely physical nature. The structure of the zeolite stays intact during the adsorption process (and its later regeneration), and dissolution effects like with other drying agents like calcium compounds cannot happen.
How do Super Dry Dynamic Dry Units work?
Our newly developed, dynamic high performance drying unit U-5002, can achieve air moisture levels of under 0.3% RH, even at temperatures of 60°C. Unlike traditional technology, during cabinet door openings, air moisture levels barely rise above 5% and sink within a few minutes again to
The desiccant is refreshed by heating and allows the trapped moisture to escape as vapor (H2O) through vents located at the back of the cabinet. Once refreshed the unit is ready to seal off the exterior venting, and re-open itself to the interior to continue dehumidification. Through a microprocessor the state of the drying substance is permanently monitored and compared with the input set point values. Only when the moisture content in the cabinet exceeds the absorption capacity of the drying substance, is the thermal regeneration started. The strength of this process in turn is fitted individually to the desired set point moisture in the cabinet. This has led to a drastic reduction of energy demand in cabinets which are infrequently opened. Often a regeneration of only 8 minutes suffices many weeks of component drying. In manufacturing environments, the regeneration behavior of the drying unit adapts itself dynamically to the needs of short opening cycles of the doors and frequent access.
Through this, the availability and efficiency is improved considerably in comparison to traditional fixed regeneration cycles.