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2 Theoretical background 11 2 Theoretical background Adsorption is the process of mass transfer from the fluid phase to the solid adsorbent, which is driven by the gradient of the chemical potential. The graphical representation of the system along with an indication of involved phases is presented in Fig. 2-1. Molecules being adsorbed lose their freedom of movement; therefore, the entropy and Gibbs free energy decrease in the system, which is consequently associated with a heat release. In accordance with Le Chatelier's principle, the adsorption process is intensified at low operating temperatures and high concentration or partial pressure of the adsorptive phase. In contrast, desorption is the reverse process to adsorption, during which the mass transfer occurs from the solid adsorbent to the bulk phase. It is an endothermic process, favoured at high operating temperatures and low adsorptive concentration. Fig. 2-1 Graphical representation of the system involving the adsorption/desorption process [51] Depending on the nature of bonding forces between adsorbed molecules and active sites located on the surface of the adsorbent, the deposition of adsorptive occurs according to physisorption or chemisorption. Physisorption is associated with intermolecular forces (van der Waals forces) and does not involve a significant change in the electronic orbital patterns of the subjected molecules [52]. This process is reversible and its adsorption enthalpy is low (20-40 kJ/mol). Moreover, adsorbed molecules can form multilayers on the adsorbent surface. On the other hand, chemisorption is associated with valence forces of the same kind as those operating in the formation of chemical compounds [52]. It is an irreversible process of a high adsorption enthalpy (80-240 kJ/mol), during which a monolayer is formed on the adsorbent surface. 2.1 Adsorption equilibrium Adsorption is induced by an imbalance imposed from the surroundings of the system [51]. Therefore, if the adsorbent is exposed to an adsorptive atmosphere, the system strives for thermodynamic equilibrium, which is energetically most favourable [53]. At the equilibrium state, the rate of adsorption and desorption is equal; namely, the net amount adsorbed does not change over time. However, once an imbalance is introduced to the system, the state variables adapt to achieve the energetic optimum yet again.PDF Image | Modelling and Simulation of Twin-Bed Pressure Swing Adsorption Plants
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