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      Adsorption Process

      Adsorption is a physical-chemical phenomenon in which the gas is concentrated on the surface of a solid or liquid to remove impurities. Typically, carbon is the adsorbing medium, which can be regenerated upon desorption, and the quantity of material adsorbed is proportional to the surface area of the solid. Consequently, adsorbents are usually granular solids with a large surface area per unit mass. Subsequently, the captured gas can be desorbed with hot air or steam, either for recovery or for thermal destruction.

      Adsorbers are widely used to increase a low gas concentration prior to incineration unless the gas concentration is high in the inlet air stream. Adsorption is also employed to reduce problem odors from gases. There are several limitations to the use of adsorption systems, but it is generally felt that the major one is the requirement for minimization of particulate matter and/or condensation of liquids (e.g., water vapor) that could mask the adsorption surface and drastically reduce its efficiency. Absorption differs from adsorption, in that it is not a physical-chemical surface phenomenon, but an approach in which the absorbed gas is ultimately distributed throughout the absorbent (liquid). The process depends only on physical solubility and may include chemical reactions in the liquid phase (chemisorption). Common absorbing media used are water, aqueous amine solutions, caustic, sodium carbonate, and nonvolatile hydrocarbon oils, depending on the type of gas to be absorbed. Usually, the gas-liquid contactor designs that are employed are plate columns or packed beds.

      Separation by adsorption chromatography essentially commences with the preparation of a porous bed of finely divided solid, the adsorbent. The adsorbent is usually contained in an open tube (column chromatography). The sample is introduced at one end of the adsorbent bed and induced to flow through the bed by means of a suitable solvent. As the sample moves through the bed, the various components are held (adsorbed) to a greater or lesser extent depending on the chemical nature of the component. Thus, those molecules that are strongly adsorbed spend considerable time on the adsorbent surface rather than in the moving (solvent) phase, but components that are slightly adsorbed move through the bed comparatively rapidly.

      Numerous factors randomly affect the process of migration through a bed, and in fact, the total distance traveled in a given time by different molecules of the same material is not constant. Nevertheless, the suitable choice of a bed and a moving (solvent) phase allows adequate separation of even multi-component mixtures to be achieved.

      Adsorption processes for water treatment are used to remove dissolved metals, organic compounds, and many toxic substances. Carbon which has been activated (oxidized at high temperatures) to create millions of minute cavities is an effective and economical means of adsorbing dissolved organic and other chemicals that cannot be removed by filtration.

Granular activated carbon and activated carbon black are used in combination with filtration and/or disinfection to remove undesirable tastes and odors, chloramines, some pesticides and herbicides, and other chemical contaminants. They are sometimes also combined with halogen-based disinfection technologies to remove residual iodine and chlorine. Activated carbon does not remove salts.

      Adsorption with regenerated carbon slurries and with resin particles is common, and some systems use activated carbon particles that are contained in a fixed bed, either without regeneration or with regeneration within the column. In all cases, the separation involves physical adsorption of the contaminants on the surfaces of the particulate medium.

      See also: Adsorption, Adsorption Isotherm, Gas Cleaning, Gas Processing, Gas Treating.