Towards a unified thermodynamic framework for adsorption

Adsorption has become one of the most important phenomena in industrial process to design separation and purification processes. It has been heavily studied experimentally in over the past 50 years and various models have been developed to predict or capture the adsorption phenomena to obtain useful insights. Adsorption is studied in mainly two categories: Pure component adsorption and multicomponent (mixed-gas) adsorption. Adsorption is expressed in terms of isotherms and IUPAC categorizes the pure component isotherms in six types: Type-I to Type-VI. Langmuir isotherm that addresses monolayer adsorption i.e., Type-I, but fails to correlate the adsorption for wide range of pure component systems. Numerous theoretical, semiempirical, and empirical models have been proposed to represent the Type-I adsorption isotherms such as Freundlich, Sips, Toth but they either lack strong thermodynamic framework or fail to capture the isotherm data. Thermodynamic Langmuir isotherm has rigorous thermodynamic background and successfully captures the Type-I isotherm data. However, it fails to capture multilayer adsorption behavior. BET isotherm is shown to express multilayer adsorption behavior i.e., Type-II to IV, a widely accepted method often employed to calculate the adsorbent specific surface area. However, it is applicable in a specific relative pressure range and fails to address capillary condensation phenomena. This dissertation focuses on development of a unified thermodynamic framework which captures monolayer and multilayer adsorption behavior. The tBET model developed represents, monolayer adsorption, and multilayer adsorption for complete relative pressure range. Furthermore, the tBET isotherm with capillary condensation predictions captures Type-I to Type-IV isotherm for complete relative pressure range. tBET isotherm with capillary condensation predictions can be utilized further for determination of accurate adsorbent specific surface area, pore size distribution, and adsorbent specific pore volume of an adsorbent. This dissertation presents generalized Langmuir isotherm (gL), a novel thermodynamic framework for multicomponent adsorption. The gL isotherm captures multicomponent adsorption better than extended Langmuir, IAST, and RAST-aNRTL. The spreading pressure dependence of the binary interaction parameter in RAST-aNRTL can be successfully identified by utilizing gL isotherm mixed gas adsorption. Therefore, providing a connection and evidence that gL and RAST-aNRTL are different thermodynamic frameworks which represent same mixed-gas adsorption equilibria. The gL isotherm having better predictive capabilities, being faster, does not require spreading pressure calculations, and ability to predict pressure effect makes it better thermodynamic framework for mixed-gas adsorption. Therefore, gL has a better utility in modeling, design and simulation of mixed-gas adsorption processes.

Embargo status: Restricted until 06/2026. To request the author grant access, click on the PDF link to the left.