A priori predictions of type I and type V isotherms by the rigid adsorbent lattice fluid
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A priori predictions of type I and type V isotherms by the rigid adsorbent lattice fluid Maarten C. Verbraeken1 · Stefano Brandani1 Received: 16 August 2019 / Revised: 1 November 2019 / Accepted: 3 November 2019 © The Author(s) 2019
Abstract Adsorbents exhibiting non type I adsorption behaviour are becoming increasingly more important in industrial applications, such as drying and gas separation. The ability to model these processes is essential in process optimisation and intensification, but requires an accurate description of the adsorption isotherms under a range of conditions. Here we describe how the Rigid Adsorbent Lattice Fluid is capable of a priori predictions both type I and type V adsorption behaviour in silicalite-1. The predictions are consistent with experimental observations for aliphatic (type I) and polar (type V) molecules in this hydrophobic material. Type V behaviour is related to molecular clustering and the paper discusses the model parameters governing the presence/absence of this behaviour in the predicted isotherms. It is found that both the solid porosity and the adsorbate interaction energy/energy density are deciding factors for the isotherm shape. Importantly, the model, whilst thermodynamically consistent, is macroscopic and thus computationally light and requires only a small number of physically meaningful parameters. Keywords Stepped isotherms · Lattice fluid model · Adsorption thermodynamics · Type V isotherms
1 Introduction Sigmoidal isotherms, or type V by the IUPAC classification system, are widespread in adsorption systems involving polar molecules, such as water on activated carbon. The behaviour is typically attributed to molecular clustering of the adsorbate, due to weak adsorbate—adsorbent versus strong intermolecular interactions and hence unfavourable adsorption, with subsequent pore filling. This leads to an initially convex isotherm, with an inflection to become concave and showing saturation at high pressures. Indeed, this type of isotherm is mostly associated with adsorption in mesoporous adsorbents, where condensation of the adsorbate, that is, formation of bulk liquid, in the porous structure is possible. However, type V isotherms have also been observed Electronic supplementary material The online version of this article (doi:https://doi.org/10.1007/s10450-019-00174-7) contains supplementary material, which is available to authorized users. * Stefano Brandani [email protected] 1
School of Engineering, University of Edinburgh, The King’s Buildings, Robert Stevenson Road, Edinburgh EH9 3FB, UK
for a number of microporous materials, such as (silicon) aluminium phospates (ALPO and SAPO) (Henninger et al. 2010), zeolitic imidazolates, (e.g. ZIF-8) (Cousin Saint Remi et al. 2011) and metal organic frameworks (MOFs) (Küsgens et al. 2009), for which this mechanism seems somewhat unsatisfactory as by their definition the size of the micropores should preclude the existence of bulk liquid. Molecular simulations have shown that molecular clustering in micropor
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