Carbon adsorption works when simple condensation cannot be effective due to the presence of non-condensables. If a process exhausts air containing 1% toluene vapor - 10,000 parts per million-volume basis (ppmv) - the presence of the air hinders the condensation process. The saturation vapor concentration in air at 20 degrees C is 31,000 ppmv. Condensation begins at around 5 degrees C. Lowering the temperature to -27 degrees C (1-mm-Hg vapor pressure) will increase the condensation efficiency to 87% which still leaves 1300 ppmv in the exhaust air. To achieve 100 ppmv requires cooling to about -80 degrees C. This requires extensive refrigeration, and if moisture is present in the exhaust air, this will rapidly freeze the exchanger unless reversing exchangers or other costly technologies are employed.
The 1% toluene vapor concentration in air was chosen as an example because the lower explosive limit (LEL) of toluene is 12,700 ppmv and it is always preferable to operate below the explosive range. Carbon adsorption will readily capture the toluene vapor when operating at 30 degrees C; outlet will be less than 10 ppmv which is a 99.9% capture. If the moisture vapor content is less than 50% relative humidity (RH), the moisture has no impact on performance.
Other volatile organic vapors behave similarly. For instance, the saturation vapor concentration of acetone in air (or in nitrogen) at 20oC is 340,000 ppmv. If in air, this concentration is much higher than the LEL (21,500 ppmv) and some form of dilution is preferable. Cooling the process to -60 degrees C will leave about 1000 ppmv in the air. Carbon adsorption at 30 degrees C will reduce the outlets to below 100 ppmv.
Carbon is thermally or chemically activated with the objective of providing a vast surface area. Typically, 1 lb has more than 100 acres of surface, which enables activated carbon to act as a molecular sponge.