1. Produktbeschreibung und Anwendung
PORESPHERE™ AL-N is a high-purity alumina-silica gel designed for efficient moisture adsorption. The inclusion of aluminum oxide enhances its structural stability and provides slightly improved resistance to hydrolysis compared to pure silica desiccant beads. Despite this, it retains its hydrophilic properties, ensuring excellent adsorption performance.
Type N is ideal for applications requiring high mechanical stability and reliable, uniform moisture control under dry or moderately humid conditions, such as in biogas plants, gas processing, and industrial manufacturing.
Unlike PORESPHERE™ AL-W, AL-N is not recommended for direct contact with water due to its lower aluminum oxide content
1.1 Mikrostruktur und Porenstruktur
PORESPHERE™ AL-N is a specially developed, tightly porous silica gel, composed of an amorphous, three-dimensional silicon dioxide (SiO₂) network. Unlike conventional silica gel, it contains 3% aluminum oxide (Al₂O₃), which modifies its chemical composition and surface polarity.
The gel features narrow micropores (< 2 nm), predominantly populated with silanol (Si–OH) groups, and a smaller proportion of aluminol (Al–OH) groups. These highly polar surface centers enable strong physical adsorption via hydrogen bonds, even at low humidity levels, resulting in rapid and intense initial adsorption.
The formation of Si–O–Al bonds alongside typical Si–O–Si bonds provides more stable and chemically robust heteroatomic bonds, improving both hydrothermal and mechanical stability. This makes AL-N more durable, with better resistance to fracture and hydrostatic pressure.
The pore structure shift toward medium-sized pores (mesopores) slightly reduces mechanical stress within the material, enhancing its fracture resistance and stability under moderate humidity conditions. AL-N absorbs moisture more slowly but effectively, making it perfect for applications with long service lives, operating at moderate pressures and flow rates.
1.2 Adsorptionsverhalten
PORESPHERE™ Type N exhibits physical adsorption (physisorption) on its surface, aided by its aluminum oxide content. Water molecules are bound to the silica gel’s silanol groups (hydroxyl groups) through dipole-dipole interactions and hydrogen bonds. The adsorption capacity depends on the specific surface area, pore size distribution, and chemical composition.
With its low alumina content, the silanol groups remain the primary binding sites for water molecules, ensuring high adsorption activity at low to medium humidity levels. The few aluminol groups help stabilize the surface of the desiccant beads, reducing mechanical stress and improving hydrothermal stability, which prevents structural degradation in higher humidity conditions.
Thanks to its narrow pore structure, AL-N offers long service life and consistent moisture absorption, making it a reliable choice for applications requiring steady performance over time. The pore structure also differentiates AL-N from PORESPHERE™ AL-H.
Temperature dependence:
| Temperature / C° | |
| < 20 | AL-N performs optimally, with strong water molecule binding even at low humidity. |
| 20 – 60 | Performance weakens as thermal energy destabilizes hydrogen bonds, promoting desorption. |
| > 60 | The adsorption capacity drops significantly, as the narrow pore structure struggles to retain water. |
Humidity Dependence:
| Relative Humidity | Beschreibung |
| < 20%RH | Many active adsorption sites remain active, making AL-N ideal for dry air. |
| 20 – 60%RH | The material rapidly reaches its maximum capacity. |
| > 60%RH | Saturation occurs quickly. The high hydrophilicity of AL-N can cause mechanical stress on pore walls, potentially damaging the structure. |
1.3 Regeneration
The adsorption of water in silica beads is a physical, reversible process. PORESPHERE™ AL-N can be regenerated at 140°C, where the granules are heated for several hours. Temperatures below 100°C are insufficient for full regeneration, while temperatures above 180°C may damage the pore structure.
Thermal regeneration can be achieved with hot air, or with inert gases like nitrogen (N₂) or argon (Ar) in sensitive processes. In this process, the released water molecules are carried away by the gas flow, preventing re-adsorption. Inert gases also ensure contamination-free and safe regeneration, especially in processes involving flammable gases or chemicals.
Thermal regeneration is used in industrial drying systems and gas processing, where long-term material efficiency is crucial. In technical adsorbers, regeneration often occurs in alternating chambers, with one chamber adsorbing moisture while the other is heated for regeneration.
Vacuum regeneration reduces pressure in the adsorber, weakening the bond between water molecules and the pore surfaces. This process is more energy-efficient and gentle on materials, ideal for temperature-sensitive applications. However, it requires vacuum pumps and pressure-resistant systems.
