text: Adsorbents are used usually in the form of spherical pellets, rods, moldings, or monoliths with a hydrodynamic radius between 0.25 and 5 mm. They must have high abrasion resistance, high thermal stability and small pore diameters, which results in higher exposed surface area and hence high capacity for adsorption. The adsorbents must also have a distinct pore structure that enables fast transport of the gaseous vapors.
  Most industrial adsorbents fall into one of three classes:
  * Oxygen-containing compounds – Are typically hydrophilic and polar, including materials such as silica gel, limestone (calcium carbonate) and zeolites.
  * Carbon-based compounds – Are typically hydrophobic and non-polar, including materials such as activated carbon and graphite.
  * Polymer-based compounds – Are polar or non-polar, depending on the functional groups in the polymer matrix.