Gas-Phase Adsorption Isotherms
In adsorption, a dynamic phase equilibrium is established for the distribution of the solute between the fluid (gas, vapour or liquid) and the solid surface.
The equilibrium is usually expressed in terms of partial pressure (gas, vapour) or concentration (liquid) of the adsorbate in the fluid and the solute loading on the adsorbent, expressed as mass, mole or volume of adsorbate per unit mass, mole or volume of the adsorbent.
Unlike vapour-liquid and liquid-liquid equilibria, where theory is often applied to estimate phase distribution, no acceptable theory has been developed to estimate fluid-solid adsorption equilibria. Thus it is necessary to obtain experimental equilibrium data for a particular solute, or mixtures of solutes and/or solvent, and a sample of the actual solid adsorbent material of interest.
If the data are taken over a range of fluid concentrations at a constant temperature, a plot of solute loading on the adsorbent versus concentration or partial pressure in the fluid can be made. Such a plot is called the adsorption isotherm. We focused primarily on gas-phase adsorption. Liquid-phase adsorption exhibits their own isotherms as well.
For pure gases, experimental physical adsorption isotherms have shapes, that are classified into 5 types as discussed below. Each of these types is observed in practice but by far the most common are types I, II and IV.
An inherent property of Type I isotherms is that adsorption is limited to the completion of a single monolayer of adsorbate at the adsorbent surface. Type I isotherms are observed for the adsorption of gases on microporous soilds whose pore sizes are not much larger than the molecular diameter of the adsorbate. Complete filling of these narrow pores corresponds to the completion of a molecular monolayer. An example is the adsorption of oxygen on carbon black at -183 oC. Adsorption in all other types do not reach a limit corresponding to the completion of a monolayer.
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Type II isotherms do not exhibit a saturation limit as did Type I. This type of isotherm indicates an indefinite multi-layer formation after completion of the monolayer and is found in adsorbents with a wide distribution of pore sizes. Near to the first point of inflexion (point A) a monolayer is completed, following which adsorption occurs in successive layers. An example is the adsorption of water vapour on carbon black at 30 oC.
Type III isotherm is obtained when the amount of gas adsorbed increases without limit as its relative saturation approaches unity. This type of isotherm is obtained when bromine is adsorbed on silica gel at 20 oC.
Type IV isotherm is a variation of Type II, but with a finite multi-layer formation corresponding to complete filling of the capillaries. The adsorption terminates near to a relative pressure of unity. This type of isotherm is obtained by the adsorption of water vapour on activated carbon at 30 oC.
Type V isotherm is similar variation of Type III obtained when water vapour is adsorbed on activated carbon at 100 oC.
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Uses of Adsorption Isotherms
Adsorption isotherms are most commonly used to select the adsorbent or even the adsorption process as a unit operation for the adsorptive separation of gases.
If the adsorption isotherm shape is Type I, II or IV, adsorption can be used to separate the adsorbate from the carrier gas. If it is Type III or V, adsorption will probably not be economical for the separation.
Although isotherms are indicative of the efficiency of an adsorbent for a particular adsorbate removal, they do not supply data to permit the calculation of contact time or the amount of adsorbent required to reduce the solute concentration below prescribed limits.
Various expressions are available to describe the various isotherms.Click here for more information.
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