Single Gases and Vapors

Adsorption isotherms are not always concave to the pressure axis. The shapes shown in Fig. 11.3 have all been observed for various systems. Here the ordinate is plotted as equilibrium partial pressure p divided by the saturation vapor pressure p of the adsorbed substance (actually, the relative saturation) in order to place all the curves on a comparable basis. 

There are three commonly used mathematical expressions to describe vapor adsorption equilibria the Langmuir, the Brunauer-Emmett-Teller (BET), and the Freundlich isotherms [13, 18, 44, 82, 102]. All except the last were derived with a theory in mind, but none is applicable universally, nor can it be predicted which, if any, will apply to a particular case. Since we shall make no application of them, they are not detailed here. 

The curves of Fig. 11.1 are true equilibrium curves and therefore represent completely reversible phenomena. The conditions corresponding to point A on 

Flgrae 113 Reference-substance plot of equilibrium adsorption of acetone on an activated carbon. [Data of Josefemtz and Othmer, Ind. Eng. Chem., 40, 739 (1948).] 

Since adsorption is an exothermic process, die concentration of adsorbed gas decreases with increased temperature at a given equilibrium pressure, as the several acetone isotherms of Fig. 11.1 indicate. 

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