Prediction of mixed-gas adsorption equilibria and isosteric heat of adsorption from its pure component adsorption isotherms

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2020-12

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Abstract

Mixed-gas adsorption equilibria and isosteric heat of adsorption are two key variables to design adsorptive separation and purification units such as pressure swing adsorption (PSA) and temperature swing adsorption (TSA). Despite the importance of these variables, the adsorptive process design mostly depends on the experimental or pilot plant data which is very expensive and tedious process. Thus, an accurate and reliable modeling approach with a rigorous thermodynamic framework is necessary to provide a strong foundation for adsorptive process design and optimization. The main focus of this dissertation is to established a reliable and robust approach to predict the mixed-gas adsorption equilibria including very nonideal systems from its pure component adsorption isotherms. Current popular predictive models such as extended Langmuir (EL), dual-site Langmuir (DSL), and ideal adsorbed solution theory (IAST) fail to predict mixed-gas adsorption of nonideal systems. Compared to these models, the real adsorbed solution theory (RAST) is reliable for nonideal systems where adsorbed phase nonideality is expressed in terms of an activity coefficient model such as adsorption non-random two liquid (aNRTL) or empirical spreading pressure dependent model. However, the adsorbed phase activity coefficient models with RAST require binary experimental adsorption data which is rarely available in the literature compared to pure component isotherms. So, it is very important to develop a predictive method to estimate the mixed-gas adsorption equilibria from its pure component isotherms. In this dissertation, a predictive approach is discussed to predict binary and multicomponent gas adsorption equilibria from its pure component thermodynamic Langmuir (tL) isotherms. The tL isotherm model also helps to predict the isosteric heat of adsorption of pure and mixed-gas adsorption. The predicted isosteric heat of adsorption from tL model reliably explains the energetic heterogeneity of adsorbent surface for monolayer gas adsorption. The accurate prediction of isosteric heat of adsorption and mixed-gas equilibria calculation mostly depends on the quality of pure component isotherms. Unfortunately, experimental data reproducibility is another major issue in adsorption literature. Generally, pure component experimental isotherm data are affected by measurement techniques, moisture, adsorbent impurity, activation temperature etc. So it is very important to assess the quality of isotherm data and predict reliable isotherm parameters. In this regards, proper choice of isotherm model, isotherms data at a wide temperature and pressure, and isosteric heat of adsorption are very helpful to estimate a reliable set of isotherm parameters. The prediction of mixed-gas adsorption equilibria is further extended to predict the surface excess of liquid adsorption. Using vapor-liquid equilibria and mixed-gas adsorption equilibria calculations, it is possible to predict the surface excess of liquid adsorption. In summary, this dissertation will give some fundamental guidelines for the reliable prediction of mixed-gas adsorption equilibria and isosteric heat of adsorption from its pure component tL adsorption isotherms.


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Adsorption, Thermodynamics, Mixed-gas adsorption equilibria, Heat of adsorption

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