High-Accuracy PVT Relationships for Compressed Fluids and Their Application to BET-like Modelling of CO₂ and CH₄ Adsorption

Abstract

This paper presents a set of high-accuracy formulae enabling the evaluation of the compression factor, activity and cohesion energy of fluids at near-critical and super-critical temperatures, with reduced molar volumes ranging from 0.4. Such data are necessary in BET-like theoretical modelling of adsorption process.The proposed fluid state equation (PVT relationship) combines a theoretical description of the fluid entropy (based on a hard-sphere model) with cohesion energy relationships obtained via high-accuracy approximations of universal compression factor data and vapour-liquid equilibria. The resultant relationships are incorporated into a BET-like description of adsorption in materials of irregular microporous structure (LBET model).The set of formulae gathered together in this paper allows the possible effective calculation of adsorption isotherms at near-and super-critical temperatures relative to the pore structures. A multi-variant fitting of the LBET model to the empirical data is proposed to detect active constraints for multilayer adsorption and, hence, to obtain information on the structure of the pores.Application of the formulae to the analysis of methane and carbon dioxide adsorption onto an active carbon is discussed.

A qualitative approach to adsorption mechanism identification on microporous carbonaceous surfaces

“…(13) T k (V mpi , p = 0), T k (V mpi , p) and Eq. (24) T t (V mpi , p), with the thermal expansion curve found with (Milewska-Duda and Duda 2009). This comparison brings strong confirmation of the adequacy of the proposed approach.…”

“…where d c is a solubility parameter and E vap is evaluated with PVT relationship for V m , a molar volume of adsorbate in its reference state (for vaporous substances it is the liquid state), see (Milewska-Duda and Duda 2009). In order to get an insight into adequacy of the kinematic evaluations in Eq.…”

Effect of adsorbate properties on adsorption mechanisms: computational study

“…chemical heterogeneity is neglected), predefining geometrical heterogeneity in the form of cavity of 0.7 nm depth and diameter: at the top layer 1.1-1.6 nm and at the second layer 0.8-1.9 nm Adsorption (2016) 22:589-597 593 temperatures T t V m pi ; p À Á , Eq. (12) with appropriate adsorbate liquid thermal expansion curve, evaluated with formulae in (Milewska-Duda and Duda 2009). In our previous paper it was shown (Ziółkowska and Duda 2015), that graphite sample model gives results of T t almost in perfect agreement with nitrogen expansion curve, enabling to conclude that for such structures the classical localized adsorption assumption (i.e.…”

Application of the LBET class models with the original fluid state model to an analysis of single, double and triple carbon dioxide, methane and nitrogen adsorption isotherms

“…is the effective intermolecular contact surface ratio, aa is the value for adsorbate-adsorbate contacts, C is the averaged value for all adsorbate-pore contacts at the second and higher layers, A for adsorbate-pore contacts at the first layer, and Q C is the adhesion energy calculated with the Berthelot rule [46][47][48].…”

“…To make possible application of the LBET equations for near and supercritical temperatures an original fluid state model was elaborated [48], fitting (with high accuracy) compression factor data [50]. The model is based on the following expression:…”