I. Introduction
Analysis of gas-solid physical adsorption equilibrium is important to design
separation and purification processes as well as heterogeneous chemical
reactors. The equilibrium between the fluid phase and the adsorbent phase
is expressed by adsorption isotherms. The first classification of physical
adsorption isotherms was presented by Brunauer et al1.
In 1985 the IUPAC Commission on Colloid and Surface Chemistry proposed
modification of this classification by addition to the original five types
of Brunauer et al.a six type, the stepped isotherm2.
Type I (the Langmuir isotherm) is typical of microporous adsorbents (activated
carbons, zeolites). Next two are typical of nonporous materials with strong
(type II) and weak (type III) fluid-surface forces. Types IV and V are
characteristic for mesoporous materials when capillary condensation occurs
(these types exhibit hysteresis loop). Type VI occurs for materials with
relatively strong fluid-surface forces, usually when the temperature is
near the melting point for the adsorbed gas.
Many theories and models have been presented in the literature to describe
these different types of isotherms. Gregg and Sing 3
have given a detailed discussion of the various models used to interpret
each type of the isotherms.
The derivation of a scientifically based adsorption isotherm was first
achieved by Langmuir (1918). The Langmuir isotherm model assumes monolayer
adsorption on a homogeneous surface. Originally based on a kinetic basis,
the model is thermodynamically consistent as it reduces to Henry's law
at low pressures. The model was also derived based on statistical thermodynamics.
Although the Langmuir isotherm was introduced more than 80 years ago, it
still remain the most commonly used adsorption isotherm equation. Its success
undoubtedly reflects its ability to fit a wide variety of adsorption data
quite well, but it may also partly reflect the appealing simplicity of
the isotherm equation and the ease with which its adjustable parameters
can be estimated.
By using more realistic assumptions, many modifications have been made
to the Langmuir isotherm, which are well summarized in the monographs of
Ruthven4,
Yang5,
Adamson6,
Rudzinski and Everett7.
The purpose of this study is to propose some generalization of Langmuir
isotherm, which will give the possibility to extend the description for
multilayer adsorption.
