Chapter 2. Application of lattice-Gas models to describe mixed-gas adsorption equilibria on heterogeneous solid surfaces

Abstract

In order to describe the separation and purification of gases and liquids by adsorption [1,2], heterogeneous catalysis [3] and many similar processes, it is indispensable to describe the adsorption equilibria of mixtures of components on real solid surfaces. Early experimental data on the adsorption of mixtures were reported in papers [4,5]. Recent papers discuss a large number of binary adsorption systems [6,7]. Most of those data relate to the adsorption of hydrocarbon mixtures by activated carbons, silica gels and zeolites. A generalization of the Langmuir model for the adsorption of mixtures on homogeneous surfaces was given by Markham and Benton [8] (see also [9–11]). The energetic heterogeneity of real surfaces and the lateral interactions between adsorbed molecules can result in considerable deviations from strictly ideal behaviour of adsorbed phases on homogeneous surfaces. Calculations of adsorption equilibria are connected with a search for the optimum conditions for carrying out separation processes over a wide range of temperatures, total pressures and partial pressures of various components. In a similar way as for one-component systems, an increase in surface coverage results in a corresponding increase of the role of the cooperative effects and of the related processes of local clustering and macroscopic ordering of the adsorbed molecules. To describe real adsorption systems it is necessary to simultaneously calculate the cooperative effects and the heterogeneity of the adsorbent surface. At present, the theory of adsorption equilibrium for gas mixtures based on the lattice-gas model is elaborated at the same level as for one component systems. The equations obtained for adsorption equilibria can serve as a basis for the verification of the applicability of the simplified equations which are widely used in the description of experimental data [12,13]. Below, we will discuss the basic results obtained for the case of approximately equal sizes of the components of the mixture. This is the most commonly used assumption (explicitly or otherwise) in most papers on the adsorption of mixtures. Rejection of this assumption highly complicates the description of the adsorption equilibria. This problem is briefly considered in Section 6. Special emphasis is laid on the molecular basis of the theories and on the physical meaning of the simplifications used. Before considering the influence of surface heterogeneity and lateral interactions between adsorbed particles (Section 3), peculiarities of the adsorption of mixtures on heterogeneous surfaces are considered in the absence of lateral interactions (Section 1) and on homogeneous surfaces but including lateral interactions (Section 2). Multilayer adsorption of mixtures on non-porous adsorbents, and in porous materials is also considered in Section 4. The equations for the partial adsorption isotherms are used to calculate the mixture separation coefficient (Section 5).