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these factors varies from system to system. In the great number of PSA processes the selectivity depends on differences in the equilibrium but there are also some PSA processes in which the separation depends on the differences in the adsorption rate [10]. 2.3 Isotherms In order to obtain a perception of the adsorption mechanism, adsorption isotherms are employed. They consist in curves that relate the holding of a substance from a fluid phase in the solid-phase at equilibrium. Due to adsorption complexity, several models of isotherms have been developed [11]. The shape of the isotherm is very important when deciding on the sorbate that will be used in a process. Isotherms can be categorized as linear, favourable or unfavourable. When the isotherm has a convex upward curvature it is entitled as favourable, and the dimensionless adsorbed phase is always higher than the dimensionless fluid phase concentration. When an isotherm is favourable in the adsorption step it is unfavourable in the desorption, once that the initial and final states are reversed and vice-versa [12]. Figure 2.1 – Dimensionless equilibrium isotherm showing the meaning of “favorable”, “linear” and “unfavorable” Several isotherms can be found in the literature and each can produce a better fitting for a specific situation. The Langmuir isotherm approaches the Henry’s Law for low concentrations and a saturation limit at high concentrations. The Langmuir-Freundlich isotherm combines the form of the Langmuir isotherm with the form of the Freundlich isotherm. This isotherm can be employed for several different systems and in a wide range of conditions; however, it does not reduce to Henry’s Law in the low-concentration limit [10]. Other isotherm models can be found in the literature. For example, in the work of Bastos- Neto et al. for the study of the breakthrough curves of methane, the adsorption equilibrium is described by the Toth model [13]. In 2012 Filipe Lopes et al. employed the Virial adsorption isotherm model in their work on fast-cycling VPSA for hydrogen purification [14]. Pressure Swing Adsorption for Hydrogen Purification c Linear Favorable Unfavorable Contextualization and state of art 4 qPDF Image | PRESSURE SWING ADSORPTION FOR THE PURIFICATION OF HYDROGEN
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