Kinds of sorbents based on carbon and their properties

Abstract

Most of industrial adsorption processes are based on selective absorption of specific components of the gas-vapor mixture flow. During the absorption of gas or vapor, adsorption capacity depends on the type of sorbent, its porous structure, the nature of the substance absorbed is its partial pressure and temperature [1]. In real process of the purification and separation of gases, influence of adsorption of bulk gas and other impurities, and kinetic factors may cause the need to make adjustments in calculation of the adsorption capacity, which was initially determined by isotherms of pure components. However, in all real adsorption process, curve of thermodynamic equilibrium is the main comparative characteristics of different types of adsorbents and it determines the choice of optimal operating conditions of the process. Simultaneously, the adsorption isotherm is a source of information about the structure of the adsorbent, adsorption heat and several other physic-chemical and technological characteristics. C. Brunauer [1] highlighted five main types of adsorption isotherms, which are presented in Pic. 1. In the case of technical adsorbents, type I can be characterized as microporous adsorbents that contain virtually no transient pores. Initial bulging area of type II and IV isotherms indicate presence of macro pores with more or less substantial amount of micro pores in conjunction. Less steep initial ascent isotherm curves can be explained by mono- and multimolecular adsorption only for adsorbents with transitional porous type. Initial curved section isotherms types III and V, which are rarely found, are common for adsorbent-adsorbate systems, when interaction of molecules of adsorbate with an adsorbent much less than intermolecular interaction between adsorbate molecules, for example, caused by a presence of hydrogen bonds. The first fundamental equation of adsorption isotherm was Langmuir’s equation. It is based on the assumption that the adsorption localized and occurs at active centers with the equal energy. They situated relatively rarely on the surface of the adsorbent. Consequently, the interaction between adsorbed molecules is absent. Each active center can adsorb only one molecule. According to this theory, with increasing of pressure, the part of solid surface, which is covered with molecules of adsorbate, increase. After reaching saturation pressure throughout the surface a monolayer of adsorbate is formed [2,3]. The equation of Langmuir, and hence the method for determining the surface area can be applied to systems in which the process is not complicated by multimolecular adsorption, adsorption in micro pores and capillary condensation. To such systems can be attributed the case of adsorption of gases at temperatures above the critical on nonporous or adsorbents with large pores. Despite this limitation, the equation of Langmuir commonly used in technical adsorption. Brunauer, Emett and Teller [4,5] in justifying theory multimolecular adsorption accepted that, despite the change in the total process model, the behavior of each adsorbed layer separately consistent with the concept Langmuir, adsorption localized and occur in the absence of interaction between the molecules of adsorbate. Each adsorbed layer generally obeys the Langmuir’s equation. To create the equation of multimolecular adsorption, the authors started from the point that the rate of condensation of molecules on a clean surface is equal to the evaporation rate on the first layer. Similar assumptions made when comparing the rate of condensation in each of the previous and the evaporation rate in each subsequent layer.