An isotherm model for adsorption of gases and vapours on heterogeneous adsorbents

Abstract

The adsorption of gases and vapours on heterogeneous adsorbents is generally described by assuming that the surface of an adsorbent consists of several energetic sites or patches. The adsorption on a site or a patch can be described by a local isotherm, such as the Langmuir, BET or Jovanovich equations, whereas the heterogeneity or the energy distribution on the adsorbent is expressed by a probability density function such as normal distribution, gamma distribution and exponential distribution. The overall adsorption isotherm is then derived by summing the contributions of all sites. Although the parameters of the energy distribution are generally related to the heat of adsorption, the overall adsorption isotherm is often insensitive to the parameters of the probability density functions, which are assumed on statistical grounds. In the present work, a new isotherm model is derived for the adsorption of gases and vapours on heterogeneous adsorbents, which combines the Jovanovich local isotherm and an exponential-six type energy potential function. The new model can correlate both Type I and Type II isotherms and has been tested successfully with the literature data for the adsorption of various gases and vapours on several different heterogeneous adsorbents. The absolute average error for most of the systems is below 3%. A comparison of the present model with other models for heterogeneous surfaces is presented.