Alumina is more correctly known as aluminum III oxide. Porous aluminas are produced from sintering the material at elevated temperatures; alumina in a pellet form is widely used in science and industry to trap out unwanted water and other vapors. Common industrial applications for alumina vapor traps are compressors and vacuum pumps. Use of these traps allows air or gases at the required mass flow rates to pass through the trap to the device and, importantly, the air is supplied dry and free from vapor contamination.
The alumina manufacturing process allows for some control of the porosity of these beads and their resultant surface area. It is the porosity and surface area characteristics of the alumina bead which makes it useful as a vapor trap. These materials typically have surface areas of hundreds of square meters per gram as measured by the BET surface area method. The large available surface area efficiently traps out vapors from the gas stream and adsorbs or "bonds" them within the alumina.
With time and use, the alumina becomes less efficient as the surface area decreases and porosity is lost. Attrition between the pellets can also lead to their breakdown in the trap, resulting in restricted air flow and loss of efficiency.