Generalized Langmuir Isotherm Addresses Spreading Pressure Dependency of Modified Ideal Adsorbed Solution Theory

This work addresses spreading pressure dependency in modified Ideal Adsorbed Solution Theory (IAST) coupled with Adsorption Non-random Two-Liquid (aNRTL) activity coefficient model to estimate mixed-gas adsorption equilibria. We introduce a spreading pressure dependent correlation upon the aNRTL binary interaction parameters to satisfy thermodynamic consistency and the limiting spreading pressure conditions imposed by modified IAST. We further predict mixed-gas adsorption equilibria as functions of system pressure with Generalized Langmuir (gL) isotherm mixed-gas adsorption model and the estimations are used as simulated data at various spreading pressures to identify the spreading pressure dependency of the binary interaction parameter. Based on the concept of competitive adsorption on an adsorbent with constant surface area, gL accounts for surface loading effect explicitly by treating adsorbent surface as an integral part of the adsorption system without the burden of computationally expensive spreading pressure calculations. Both gL and the modified IAST-aNRTL model with spreading pressure dependency are thermodynamically consistent and accurately capture mixed-gas adsorption equilibria including Raoult’s law ideal systems, modified-Raoult’s law non-ideal systems, and highly non-ideal azeotropic systems.