Activated alumina is produced by heating aluminum hydroxide to remove its crystal water. Its surface is rich in hydroxyl groups (–OH) and exposed aluminum ions (Al³⁺), which gives it high surface polarity and strong hydrophilicity. Through hydrogen bonding and electrostatic interactions, it preferentially adsorbs polar species such as water (H₂O), fluoride ions (F⁻), and acidic gases like SO₂. Activated alumina cannot effectively adsorb oil mainly because its surface chemistry does not match non-polar oils, and because many oil molecules are too large.
Many oil molecules cannot enter the smaller, stronger-adsorbing micropores of activated alumina at all. They can only physically adhere to the particle exterior or in macropores, and this adsorption is very weak and unstable.
In compressed-air drying systems, molecular sieves and activated alumina are preferred adsorbents thanks to their excellent water uptake and regenerability. They effectively remove moisture and can be cycled by heat regeneration. However, if oils are present in the system, such as lubricants or oil vapor, they cause serious harm. Oils adhere to the porous surface, gradually blocking the pore structure. More critically, at normal regeneration temperatures the oil contamination is difficult to volatilize and remove, creating irreversible fouling inside the adsorbent. This directly raises the pressure-dew-point of the outlet air and sharply degrades drying performance.
To ensure long-term stable operation and deep drying, the oil content of compressed air entering the dryer must be strictly controlled. This is typically achieved by two core measures: first, address the source by using an oil-free compressor; second, install efficient pre-filtration before the dryer, such as coalescing filters and activated-carbon filters, to thoroughly remove liquid oil mist and oil vapor.
Post time: Sep-08-2025