Molecular Sieves

Molecular sieves are drying agents with very specific properties. They are synthetically produced aluminosilicates. They are differentiated on the basis of their crystal lattice structure, as this results in pores of different sizes. Zeolites of type A contain sodium in their crystal lattice, thus the pore diameter is approx. 0.4 nm (pores 4A). Zeolites of structure X have a pore diameter of 0.9nm (9A). The constant structure of the crystal lattices allows a very high internal surface area of up to 1000 m²/g. Together with high electrostatic adsorption forces, these molecular sieves are therefore ideally suited for air drying.

Even at low relative humidity, molecular sieves achieve a higher absorption capacity. Thus, they are able to dry the supply air very strongly. This makes their use sensible wherever the degree of drying is a decisive selection criterion. The maximum water absorption in a fully saturated environment is approx. 23% (molecular sieve 4A) up to 27% (molecular sieve 13X). There is no colour indicator to show the loading status.

The regeneration temperature of the molecular sieve is 300°C.

Molecular sieves 3A

Molecular sieves 3A are type A desiccants and have 1x n-modules, which means that these zeolites have the highest polarity and preferentially absorb water. They are synthetically produced aluminosilicates. Type 3A zeolites contain potassium in their crystal lattice (K6Na6[(AlO2)12(SiO2)12]). The resulting pore diameter is about 0.3 nm. This constant structure results in a very high internal surface area of up to 1000 m²/g. Together with high electrostatic adsorption forces, these molecular sieves are therefore ideally suited for air drying. The maximum water absorption in a fully saturated environment is approx. 21%.

Molecular sieves 4A

Molecular sieves 4A is the most commonly used zeolite for drying air. This type A desiccant has 1x n-modules, which means that these zeolites have the highest polarity and preferentially absorb water. They are synthetically produced aluminosilicates. Type 4A zeolites contain sodium in their crystal lattice (Na12[(AlO2)12(SiO2)12]). The resulting pore diameter is about 0.42 nm. This constant structure results in a very high internal surface area of up to 1000m²/g. Together with high electrostatic adsorption forces, these molecular sieves are therefore ideally suited for air drying. The maximum water absorption in a fully saturated environment is approx. 23%.

Molecular sieves 5A

Molecular sieves 5A are desiccants of type A and have 1x n-modules, which means that these zeolites have the highest polarity and preferentially absorb water. They are synthetically produced aluminosilicates. Type 5A zeolites contain calcium in their crystal lattice (Ca4.5Na3[(AlO2)12(SiO2)12]). The resulting pore diameter is about 0.5 nm. This constant structure results in a very high internal surface area of up to 1000m²/g. Together with high electrostatic adsorption forces, these molecular sieves are therefore ideally suited for air drying. The maximum water absorption in a fully saturated environment is approx. 23%.

Molecular sieves 13X

Molecular sieve 13X are desiccants of type X and have 1-2x n-modules. As a result, the polar forces are somewhat weaker and these zeolites selectively absorb non-polar organic substances in addition to water. They are synthetically produced aluminosilicates. Type 13X zeolites contain sodium in their crystal lattice (Na86[(AlO2)86(SiO2)106]). The resulting pore diameter is about 1.0 nm. This constant structure results in a very high internal surface area of up to 1250m²/g. Together with high electrostatic adsorption forces, these molecular sieves are therefore ideally suited for air drying. The maximum water absorption in a fully saturated environment is approx. 27%.

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Chemical properties

Chemical name: Sodium aluminum silicate
CAS number: 1318-02-1

Molecular Sieve A3Molecular Sieve A4Molecular Sieve A5Molecular Sieve X13
StructureAAAX
CationsK+Na+Ca+Na+
Actual pore size0,3 nm0,42 nm0,5 nm0,74 nm
Effective pore size0,38 nm0,42 nm0,5 nm0,9 – 1,0 nm
Appearance and shapebeige, solid ballsbeige, solid ballsbeige, solid ballsbeige, solid balls
Particle size3 – 5 mm3 – 5 mm3 – 5 mm3 – 5 mm
Bulk density0,67 kg/l0,67 kg/l0,67 kg/l0,60 kg/l
Pore volume0,35 – 0,70 ml/g0,35 – 0,70 ml/g0,35 – 0,70 ml/g0,35 – 0,70 ml/g
Breaking strength>70 N>80 N>80 N>60 N
Specific surface500 – 1000 m²/g500 – 1000 m²/g500 – 1000 m²/g650 – 1250 m²/g
575°C Loss on ignition<1,5 %<1,5 %<1,5 %<2,0 %
Rate of abrasion<0,25 %<0,25 %<0,25 %<0,2 %
Water absorption capacity>210 ml/kg>230 ml/kg>230 ml/kg>270 ml/kg
Regeneration temperature300°C300°C300°C300°C

Color indicator

Molecular sieves have no color indicator. The loading condition can be measured via the change in weight or with the use of a sensor.

You can also mix molecular sieves with silica gel to indicate the loading state. However, we would like to point out that the mixture cannot be regenerated afterwards. The regeneration temperature of silica gel with color indicator is approx. 120°C – a temperature at which molecular sieve does not yet show any regeneration. At temperatures of 300°C, which in turn would be necessary for the regeneration of the molecular sieve, the silica gel would be destroyed.

No color change with molekular sieves

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