Density functional theory for the description of spherical non-associating monomers in confined media using the SAFT-VR equation of state and weighted density approximations

As a first step of an ongoing study of thermodynamic properties and adsorption of complex fluids in confined media, we present a new theoretical description for spherical monomers using the Statistical Associating Fluid Theory for potential of Variable Range (SAFT-VR) and a Non-Local Density Functional Theory (NLDFT) with Weighted Density Approximations (WDA). The well-known Modified Fundamental Measure Theory is used to describe the inhomogeneous hard-sphere contribution as a reference for the monomer and two WDA approaches are developed for the dispersive terms from the high-temperature Barker and Henderson perturbation expansion. The first approach extends the dispersive contributions using the scalar and vector weighted densities introduced in the Fundamental Measure Theory (FMT) and the second one uses a coarse-grained (CG) approach with a unique weighted density. To test the accuracy of this new NLDFT/SAFT-VR coupling, the two versions of the theoretical model are compared with Grand Canonical Monte Carlo (GCMC) molecular simulations using the same molecular model. Only the version with the “CG” approach for the dispersive terms provides results in excellent agreement with GCMC calculations in a wide range of conditions while the “FMT” extension version gives a good representation solely at low pressures. Hence, the “CG” version of the theoretical model is used to reproduce methane adsorption isotherms in a Carbon Molecular Sieve and compared with experimental data after a characterization of the material. The whole results show an excellent agreement between modeling and experiments. Thus, through a complete and consistent comparison both with molecular simulations and with experimental data, the NLDFT/SAFT-VR theory has been validated for the description of monomers.This work was sponsored by the ERC advanced grant Failflow (27769). This financial support is gratefully acknowledged. This work was supported by Acción Integrada España-Francia from Ministerio de Ciencia e Innovación and Picasso Project (Project Nos. FR2009-0056 and PHC PI- CASSO2010). F.J.B. would like to acknowledge financial support from Ministerio de Ciencia e Innovación (Project No. FIS2010-14866), Junta de Andalucía, and Universidad de Huelva. C. Malheiro would like to acknowledge the ISIFOR Carnot institute for her mobility grant

Revisiting poromechanics in the context of microporous materials

International audiencePoromechanics offers a consistent theoretical framework for describing the mechanical response of porous solids, fully or partially saturated with a fluid phase. When dealing with fully saturated microporous materials, which exhibit pores of the nanometre size, aside from the fluid pressure acting on the pore walls additional effects due to adsorption and confinement of the fluid molecules in the smallest pores must be accounted for. From the mechanical point of view, these phenomena result into volumetric deformations of the porous solid: the so-called "swelling" phenomenon. The present work investigates how the poromechanical theory should be refined in order to describe adsorption and confinement induced swelling in microporous solids. Firstly, we report molecular simulation results that show that the pressure and density of the fluid in the smallest pores are responsible for the volumetric deformation of the material. Secondly, poromechanics is revisited in the context of a microporous material with a continuous pore size distribution. Accounting for the thermodynamic equilibrium of the fluid phase in the overall pore space, the new formulation introduces an apparent porosity and an interaction free energy. We use a prototype constitutive relation relating these two quantities to the Gibbs adsorption isotherm, and then calculate the induced deformation of the solid matrix. Agreement with experimental data found in the literature is observed. As an illustrating example, we show the predicted strains in the case of adsorption of methane on activated carbon

Molecular dynamics and thermodynamical modelling using SAFT-VR to predict hydrate phase equilibria : application to CO2 hydrates

This work was dedicated to the prediction of the three phase coexistence line (CO2 hydrate–liquid H2Oliquid/vapour CO2) for the H2O+CO2 binary mixture by using (i) molecular dynamics simulations, and (ii) the well known van der Waals-Platteeuw (vdWP) model combined with the SAFT-VR equation of state. Molecular dynamics simulations have been performed using the simulation package GROMACS. The temperature at which the three phases are in equilibrium was determined for different pressures, by using direct coexistence simulations. Carbon dioxide was modelled as a linear-rigid chain molecule with three chemical units, the well-known version TraPPE molecular model. The TIP4P/Ice model was used for water. To perform the thermodynamical modelling, the SAFT-VR EOS was incorporated in the vdWP framework. The values of the cell model parameters were regressed and discussed together with the influence of some assumptions of the vdWP model. Since SAFT-VR can describe most of fluids involved in hydrate modelling (inhibitors, salts…), this study is a first step in the description of hydrate forming conditions of more complex systems. Finally, the three-phase coexistence temperatures obtained with both simulations and theory at different pressures were compared with experimental result

Phase equilibrium properties of CO 2 /CH 4 mixed gas hydroquinone clathrates: Experimental data and model predictions

Hydroquinone (HQ) clathrates seem to be promising inclusion compounds for selective CO2 capture from gas mixtures. However, to date no phase equilibrium data are known in literature for mixed-gas HQ clathrates. This study presents experimental equilibrium pressures obtained within a range of 298–343 K for different CO2/CH4 gas mixtures. The clathrate composition is given for each equilibrium point. The capture selectivity is calculated from the molar composition of the CO2/CH4 gas mixture in the clathrate and in the gas phase. The results obtained reveal that CH4 molecules in the CO2/CH4 mixtures are preferentially captured at equilibrium conditions. Our experimental data are compared against numerical predictions obtained from thermodynamic modeling using the Conde’s model. Very good agreement is found between the calculated and experimental data in terms of clathrate phase equilibria

CO2 Capture and Storage by Hydroquinone Clathrate Formation: Thermodynamic and Kinetic Studies

Hydroquinone (HQ) can form a gas clathrate in specific pressure and temperature conditions in the presence of CO2 molecules. This study presents experimental data of clathrate phase equilibrium and storage capacity for the CO2-HQ system in the range of temperature from about 288 to 354 K. Intercalation enthalpy and entropy are determined using the obtained equilibrium data and the Langmuir adsorption model. On a kinetic point of view, CO2-HQ clathrate formation by solid/gas reaction revealed a non-negligible effect of textural parameters on enclathration rate

Experimental Determination of Phase Equilibria and Occupancies for CO2, CH4, and N2 Hydroquinone Clathrates

Hydroquinone (HQ) forms organic clathrates in the presence of various gas molecules in specific thermodynamic conditions. For some systems, clathrate phase equilibrium and occupancy data are very scarce or inexistent in literature to date. This work presents experimental results obtained for the CO2–HQ, CH4–HQ, and N2–HQ clathrates, in an extended range of temperature from about 288 to 354 K. Formation/dissociation pressures, and occupancies at the equilibrium clathrate forming conditions, were determined for these systems. Experiments showing the influence of the crystallization solvent, and the effect of the gas pressure on HQ solubility, were also presented and discussed. A good agreement is obtained between our experimental results and the already published experimental and modeling data. Our results show a clear dependency of the clathrate occupancy with temperature. The equilibrium curves obtained for CO2–HQ and CH4–HQ clathrates were found to be very close to each other. The results presented in this study, obtained in a relatively large temperature range, are new and important to the field of organic clathrates with potential impact on gas separation, energy storage, and transport

Characterisation of the confinement state of a fluid in a nanometric split pore by density functional theory in the context of adsorption-induced swelling in microporous media

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我國會計師績效與社會期望之差距研究

International audienceMany nanoporous solids (e.g. activated carbons, zeolites, MOFs) can exhibit non-monotonic deformation upon adsorption, alternating between contraction and expansion stages according to the thermodynamic conditions and pore sizes. The case of water is particularly interesting, not only for its importance in many industrial applications and natural processes, but also because the adsorptive behaviour of this latter is complex and presents varied mechanisms. This study aims at characterising the adsorption-induced pore pressure of water in nanoscopic slit pores by molecular non-local density functional theory (DFT). The DFT is coupled with the statistical associative fluid theory for potential of variable range (SAFT-VR) which is highly accurate for the modelling of the bulk phase and equilibrium thermodynamic properties of water. This SAFT-DFT coupling has already been applied to methane confined in graphitic slit-like nanopores. While retaining the accuracy of molecular simulations at pore scale, the model has a very low computational cost that allowed obtaining highly resolved pore pressure maps as a function of both pore width and thermodynamic conditions. In this work, several configurations are explored for confined water by changing both the thermodynamic conditions and the pore sizes. For instance, the capillary condensation and evaporation are investigated with the theoretical model in micro- and mesopores. The adsorption hysteresis gives rise to a pressure hysteresis for which consequences on the mechanical behaviour are investigated. The influence of the surfaces activation on water adsorption and pore pressure is also explored

Estimation of the confinement state of methane and water in a nanoscopic slit pore by a density functional theory

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