Adsorption
Authors: Ruthven Douglas M. 
Published: August 2004

Adsorption is the term used to describe the tendency of molecules from an ambient fluid phase to adhere to the surface of a solid. This is a fundamental property of matter, having its origin in the attractive forces between molecules. The force field creates a region of low potential energy near the solid surface and, as a result, the molecular density close to the surface is generally greater than in the bulk gas. In a multicomponent system the composition of this surface layer generally differs from that of the bulk gas because the surface adsorbs the various components with different affinities. Adsorption may also occur from the liquid phase, although in this case there is generally little difference in molecular density between the adsorbed and fluid phases. The enhanced concentration at the surface accounts in part for the catalytic activity shown by many solid surfaces. However, most of the important applications of adsorption depend on the selectivity, difference in the affinity of the surface for different components. As a result adsorption offers a relatively straightforward means of purification and a potentially useful means of bulk separation. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals interactions. In chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the solid surface. Such interactions are both stronger and more specific than the forces of physical adsorption. Polar adsorbents such as most zeolites, silica gel, or activated alumina adsorb water more strongly than they adsorb organic species and are commonly called hydrophilic. In contrast, on a nonpolar surface water is held only very weakly and is easily displaced by organics. Such adsorbents are termed hydrophobic. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles. Carbon adsorbents are prepared by controlled burn‐out of carbonaceous materials. Like any other phase equilibrium, the distribution of a sorbate between fluid and adsorbed phases is governed by the principles of thermodynamics. In general, for physical adsorption on a homogeneous surface at sufficiently low concentrations, the isotherm should approach a linear form, and the limiting slope in the low concentration region is commonly known as the Henry's law constant. The Henry constant is a thermodynamic equilibrium constant. The isotherms for some systems, notably hydrocarbons on activated carbon, conform more closely to the Freundlich equation. In most adsorption processes the adsorbent is contacted with fluid in a packed bed. An understanding of the dynamic behavior of such systems is therefore needed for process design and optimization. The general features of the dynamic behavior may be understood without recourse to detailed calculations since the overall pattern of the response is governed by the form of the equilibrium relationship rather than by kinetics. The concentration front for adsorption will assume the form of a travelling shock wave, whereas for desorption the front will assume the form of a simple wave which spreads as it propagates through the column. In the design of a typical adsorption process the basic problem is to estimate the size of the adsorber bed needed to remove a certain quantity of the adsorbable species from the feed stream. The length of column needed for a particular duty can then be found simply by adding the length of the unused bed (LUB) to the length calculated from equilibrium considerations, assuming a shock concentration front. If the isotherm is unfavorable, the stable dynamic situation leading to constant pattern behavior can never be achieved. The applications of adsorbents are many and include the use of adsorbents in cigarette filters, in some water purification systems, as deodorants in health care products and as desiccants in storage, packaging and dual‐pane windows.