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Abstract
In this study seven adsorption azeotropes involving binary systems and zeolite-based adsorbents were systematically investigated. Pure component isotherms and mixed-gas adsorption data were taken from published literature except for the benzene–propene system on silicalite, which is newly presented in this work using molecular simulations. Experimental adsorbed phase composition and total amount adsorbed of the azeotropic systems were compared with the predictions of several models including: the ideal adsorbed solution theory (IAST), the heterogeneous ideal adsorbed solution theory (HIAST) and the real adsorbed solution theory (RAST) coupled with the 1-parameter Margules (1-Margules) and the van Laar equations. In the latter two models an additional loading parameter was incorporated in the expression of the excess Gibbs energy to account for the reduced grand potential dependency of the activity coefficients in the adsorbed phase. It was found that the HIAST and RAST–1-Margules models were able to predict the azeotropic behaviour of some systems with good accuracy. However, only the RAST–van Laar model consistently showed an average relative deviation below 3% compared to experimental data for both the adsorbed phase composition and the total amount adsorbed across the systems. This modified van Laar equation is therefore preferable in those engineering applications when the location of adsorption azeotropes is required with great accuracy and when there is lack of detailed characterization of the adsorbent that is needed to carry out molecular simulations.
Highlights
Adsorption-based processes have been recently expanding their presence in many industrial applications, due to their high selectivity, high throughput and high energy efficiency, coupled with a low maintenance and ease of operation
Whatever the cause of non-idealities, engineers require accurate thermodynamic models to correlate and predict multicomponent adsorption equilibria in support of rigorous process simulation tools. This is relevant in the design of adsorbers when it is required to locate the adsorption azeotropes with great accuracy and when there is lack of detailed characterization of the adsorbent that is needed to carry out molecular simulations
With the exclusion of x 1 for the C 3H8(1)–H2S(2)–H-modernite system, only the real adsorbed solution theory (RAST)–van Laar model consistently showed an average relative deviation below 3% compared to experimental data for both the adsorbed phase composition and the total amount adsorbed across the systems
Summary
Adsorption-based processes have been recently expanding their presence in many industrial applications, due to their high selectivity, high throughput and high energy efficiency, coupled with a low maintenance and ease of operation. They play a fundamental role in catalytic reaction processes and in several separations and purifications of gases and liquids [1,2,3]. It has been extensively reported that experimental and simulated data for mixed-gas adsorption on energetically heterogeneous surfaces such as activated carbons and zeolites can exhibit negative deviations from ideality [5,6,7]. Among the causes of non-ideal and azeotropic behaviour energetic heterogeneity of adsorption sites, steric exclusion effects and interactions between molecules in the adsorbed phase have been proposed [6, 12]
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