Adsorption is a separation process based on the ability of adsorbents to remove gas or vapor pollutants preferentially from a waste gas stream (Yang 1987; Kohl and Riesenfeld 1985). The process is particularly suitable when pollutants are (1) noncombustible, (2) insoluble in liquids, or (3) present in dilute concentration.
The mechanism of adsorption can be classified as either physical adsorption or chemisorption. In physical adsorption, gas molecules adhere to a solid surface via van der Waals forces. The process is similar to the condensation of a vapor. It is a reversible process. Desorption occurs by lowering the pressure, increasing the temperature, or purging the adsorbent with an inert gas. In chemisorp-
FIG. 5.20.9 Contact condenser.
tion, gas molecules are adsorbed by forming chemical bonds with solid surfaces. This process is sometimes irreversible. For example, oxygen chemisorbed on activated carbon can only be desorbed as CO or CO2. Chemisorbed spent adsorbents cannot normally be regenerated under mild temperatures or vacuum.
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