Encyclopedia of Renewable Energy. James G. Speight

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Adsorption

      Adsorption (sometimes referred to as physisorption) is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a film of molecules or atoms (adsorbate). It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution. Adsorption is the process by which the gas is concentrated on the surface of a solid or liquid to remove impurities; carbon is a common adsorbing medium which can be regenerated upon desorption. The term sorption encompasses both processes, while desorption is the reverse process.

      Adsorption differs from absorption in that the adsorption process is not a physical-chemical phenomenon in which the gas is concentrated on the surface of a solid or liquid to remove impurities. Usually, carbon is the adsorbing medium, which can be regenerated upon desorption. The quantity of material adsorbed is proportional to the surface area of the solid, and, 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.

      The number of steps and the type of process used to produce specification-quality gas and liquids most often depend upon the source and makeup of the wellhead production stream. In some cases, several of the steps may be integrated into one unit or operation, performed in a different order or at alternative locations, or not required at all.

      If the attractive forces are chemical in nature and result in the formation of a chemical bond between the solid adsorbent and the adsorbed molecules, the adsorption is called chemical adsorption or chemisorption. In the latter case, the removal of the chemisorbed material to regenerate the adsorbent is more difficult and may require chemical techniques to reactivate the spent adsorbent. For this reason, most adsorbents are operated as physical adsorbents with regeneration by reducing pressure, raising temperature, or flushing with solvents. However, even physical adsorbents are often slowly deactivated over many adsorption regeneration cycles by chemisorption due to unknown trace contaminants in either the adsorbent or the feed stream. At some specific deactivation level, the adsorbent must be either reactivated or discarded.

      Two general types of adsorbents are commonly used. The first, of which activated carbon is an example, has a low-energy surface and is selective for non-polar molecules. These can be hydrocarbons and other organic compounds of low polarity such as ether derivatives, ketone derivatives, ester derivatives, and halogenated hydrocarbon derivatives. This type of adsorbent is useful for recovery of organic materials from wastewaters and exhaust gases. The other type of adsorbent, which consists of activated clay minerals, alumina minerals, and silica gel, has a higher energy surface that is more selective for polar molecules.

      These materials are usually not effective for water solutions because the water selectively covers all the adsorption sites to the exclusion of everything else. However, they do find use for removing polar impurities such as water, oxidation products, or naturally occurring impurities like sulfur and nitrogen compounds from organic process streams. They are also useful for cleaning slightly contaminated waste streams to permit recycling of the major components. Zeolites and molecular sieves are special cases which depend on pore size and structure, i.e., access to the internal surface, for their selectivity rather than on adsorbent surface energy. They can be made from specially treated carbons as well as from natural or synthetic crystalline materials.

      See also: Absorption, Adsorption, Adsorption Process, Chemisorption, Gas Cleaning, Gas Processing, Gas Treating, Physisorption.

      Adsorption Isotherm

      The adsorption process is studied through the development of an adsorption isotherm which relates the amount of adsorbate (x) adsorbed on the surface of adsorbent (m) and pressure at constant temperature. Different adsorption isotherms have been developed by Freundlich, Langmuir, and by means of the Brunauer, Emmett, and Teller (BET) theory. Simply, the adsorption process can be represented as:

Freundlich Adsorption Isotherm

      Freundlich gave an empirical expression representing the isothermal variation of adsorption of a quantity of gas adsorbed by unit mass of solid adsorbent with pressure:

      In this equation, x is the mass of the gas adsorbed on mass, m, of the adsorbent at pressure p; k and n are constants whose values depend upon adsorbent and gas at particular temperature. This isotherm establishes the relationship of adsorption with lower pressures but is not always suitable for high-pressure situations.

       Langmuir Adsorption Isotherm

      The Langmuir adsorption isotherm is based on several assumptions, one of which is that dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules:

      In this equation, A(g) is the unabsorbed gaseous molecule, B(s) is unoccupied metal surface, and AB is adsorbed gaseous molecule from which a relationship between the number of active sites of the surface undergoing adsorption and pressure can be derived:

      Here, θ is the number of sites of the surface which are covered with gaseous molecule, P is the pressure, and K is the equilibrium constant for distribution of adsorbate between the surface and the gas phase. However, the Langmuir adsorption equation is that it is valid at low pressure only. At lower pressure, KP is small and the factor 1+KP in denominator is close to unity which reduces the Langmuir equation to:

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