PORESPHERE™ AL-H is a high-purity alumina–silica gel specifically engineered for efficient moisture adsorption. The targeted addition of aluminum oxide gives it a significantly stronger structure and slightly higher resistance to hydrolysis compared to pure silicon dioxide desiccant beads. Despite this modification, it retains its hydrophilic character and excellent adsorption properties.
Type H is particularly suitable for applications that demand very high mechanical stability and reliable moisture control under humid conditions—such as compressed air treatment, offshore and marine environments, or specialized industrial processes.
Unlike PORESPHERE™ AL-W, AL-H is not suitable for direct contact with liquid water due to its lower aluminum oxide content.
1.1 Microstructure & Pore Structure
PORESPHERE™ AL-H is a specially developed, wide-pore silica gel based on an amorphous, three-dimensional network of silicon dioxide (SiO₂). In contrast to conventional silica gel, it contains approximately 3.4% aluminum oxide (Al₂O₃), which selectively modifies the chemical composition and surface polarity. The material features a broadened pore structure with a significant proportion of mesopores (2–50 nm). This makes it slightly less hydrophilic than Type N while increasing its mechanical robustness.
In addition to adsorption via hydroxyl (–OH) groups, water can also be retained through capillary condensation within the larger pores
The localized formation of Si–O–Al bonds (aluminol groups) alongside the typical Si–O–Si bridges (silanol groups) results in energetically more stable heteroatomic bonds that are chemically more robust and resistant. This improves both hydrothermal and mechanical stability, making the material more durable.
The modification of the pore system leads to a shift toward medium and larger pores (meso- and macropores) within the granules. These structures reduce internal mechanical stress, enhance fracture strength and hydroresistance, and enable rapid water uptake in large quantities.
Consequently, PORESPHERE™ AL-H is ideal for demanding humidity conditions where high structural stability and reliable adsorption performance are essential—for example, in gas and compressed air drying or offshore applications. The beads can be used under high pressure and flow rates, although this may slightly reduce their service life.
1.2 Adsorption Behaviour
PORESPHERE™ AL-H adsorbs water physically (physisorption) on its surface. The water molecules bind to the silanol (hydroxyl) groups of the silica via dipole–dipole interactions and hydrogen bonds. Adsorption capacity depends strongly on the specific surface area, pore size distribution, and chemical composition.
Since the alumina content is relatively low, the silanol groups continue to play the main role in water binding. These highly polar sites attract water effectively, ensuring high activity at low to medium humidity. However, due to its wide pores, AL-H shows weaker adsorption at low humidity, which increases significantly at higher humidity levels.
The small number of aluminol groups help stabilize the bead surface, relieve mechanical stress in the pore walls, and improve hydrothermal resistance, partially preventing structural degradation at higher relative humidity.
Because of its wide-pore structure, AL-H has a high adsorption rate and can quickly absorb large amounts of water, but it also reaches equilibrium faster and therefore requires more frequent regeneration. Pore width (narrow vs. wide) is the main distinguishing feature compared to PORESPHERE™ AL-N.
Temperature dependence
| Temperature / C° | |
| < 20 | Adsorption is slightly weaker because the surface is less active at low temperatures. |
| 20 – 60 | Stable operation; adsorption performance remains consistent. |
| > 60 | High capacity maintained — larger pores help bind water through capillary condensation. |
Dependence on humidity
| Dependence on Humidity | |
| < 20%RH | Low water uptake due to fewer strongly polar sites |
| 20 – 60%RH | Gradual increase in adsorption — both surface groups and pores contribute |
| > 60%RH | Peak performance — large pores fill with water via capillary condensation without damaging the structure |
1.3 Regeneration
The adsorption of water in silica gel is a purely physical and fully reversible process, known as regeneration. PORESPHERE™ AL-H can be regenerated at approximately 140°C, maintained for several hours. Temperatures below 100°C are insufficient for complete regeneration, while those above 180°C may damage the pore structure. Depending on the application and purity requirements, different regeneration techniques can be employed.
Thermal regeneration
Thermal regeneration can be achieved using hot air (e.g., in ovens) or inert gases such as nitrogen (N₂) or argon (Ar). When heated, the adsorbed water is released from the pores and surface. Without a carrier gas, however, desorbed water molecules may re-adsorb onto the material. A continuous flow of inert gas prevents this by efficiently removing released water vapor and lowering the partial pressure inside the adsorber bed.
Inert gases are particularly beneficial when flammable gases, solvents, or sensitive chemicals are present, as oxygen could cause oxidation, decomposition, or explosion. Nitrogen and argon are chemically inert and ensure contamination-free, safe regeneration. Especially dry nitrogen, with very low absolute humidity, effectively removes water vapor from the gel.
The use of inert gases also offers advantages in processes involving flammable gases, solvents or sensitive chemicals, as oxygen could lead to oxidation, decomposition or even explosion. Nitrogen and argon are chemically inert, do not react with the silica gel or the adsorbed molecules, and thus ensure clean and safe regeneration. Particularly dry nitrogen gas with very low absolute humidity can absorb and remove water vapour extremely efficiently.
Thermal regeneration is widely used in industrial drying systems and gas treatment applications where long-term moisture control is essential. In many technical systems—such as compressed air dryers—regeneration is performed alternately in a dual-chamber setup: one chamber adsorbs while the other regenerates using a portion of the dried air. This ensures a continuous supply of dry gas without interrupting the process.
Vacuum regeneration
In vacuum regeneration, the pressure within the adsorber bed is reduced, weakening the interaction between water and the pore surface. This enables desorption even at relatively low temperatures, making the process energy-efficient and gentle on the material—ideal for temperature-sensitive systems.
The main disadvantage is higher technical complexity, as vacuum pumps and pressure-resistant systems are required.
