(72c) Thermodynamic Modeling of Adsorption Azeotropes

Adsorption is a surface process that leads to transfer of a molecule from a bulk fluid to solid surface. Recently, adsorption-based processes have been expanding their presence in many industrial applications, due to their high selectivity, high throughput, and high energy efficiency, coupled with low maintenance and ease of operation. Particularly, they play a fundamental role in catalytic reaction processes and in separations and purification of gases and liquids ^[1]. An azeotrope occurs when the gas and liquid phase mole fractions in a binary gas mixture are equal at a given temperature and pressure, that is, when they intersect the 45-degree diagonal on an x-y diagram and the selectivity becomes unity. This azeotrope formation phenomenon is commonly found in vapor-liquid equilibria and gas-solid adsorption. Numerous articles have appeared in the literature reporting on the experimental formation of adsorption azeotropes in binary gas mixtures for a variety of adsorbate−adsorbent systems ^[2]. However, the complexities of solid surfaces and our inability to characterize exactly their interaction with adsorbate molecules limit our understanding of adsorption azeotropes.

To model such behaviors, modified one-parameter Margules equation with an exponential dependency upon the reduced grand potential and similarly modified van Laar-like equation are commonly used to express the adsorbate activity coefficients representing the adsorbed phase non-ideality leading to the formation of azeotropes ^[3]. Since these equations are purely empirical and they require the calculation of hypothetical reduced grand potential, a clear understanding of this azeotrope formation phenomenon has not been fully developed so far. The study presents thermodynamic modeling of adsorption azeotropes based on the rigorous thermodynamic framework of adsorption Nonrandom Two-liquid (NRTL) ^[4] model for adsorbed phase coupled with generalized Langmuir model for multicomponent adsorption equilibria ^[5]. This comprehensive thermodynamic framework allows critical analysis for the formation of azeotrope in adsorption equilibrium at system temperature and pressure without the complication of the reduced grand potential.

References:

1. Jain, S., Moharir, A.A., Li, P., Wozny, G.: Heuristic design of pressure swing adsorption: a preliminary study. Purif. Technol. 33, 25–43 (2003).
2. Reeds, J. N.; Kammermeyer, K. Adsorption of Mixed Vapors. Eng. Chem. 1959, 51, 707−709.
3. Luberti, M., Mennitto, R., Brandani, S., Santori, G. and Sarkisov, L., 2021. Activity coefficient models for accurate prediction of adsorption azeotropes. Adsorption, 27(8), pp.1191-1206.
4. Kaur, H., Tun, H., Sees, M. and Chen, C.C., 2019. Local composition activity coefficient model for mixed-gas adsorption equilibria. Adsorption, 25(5), pp.951-964.
5. Hamid, U., Vyawahare, P., Tun, H. and Chen, C.C., 2022. Generalization of thermodynamic Langmuir isotherm for mixed‐gas adsorption equilibria. AIChE Journal, 68(6), p.e17663.