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Molecular simulation studies in the supercritical region

Parris, P.; (2010) Molecular simulation studies in the supercritical region. Doctoral thesis, UCL (University College London). Green open access

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Abstract

In our work, we employed molecular dynamics and Monte Carlo (MC) simulations to investigate the supercritical phase of carbon dioxide near its critical point. Three systems have been studied. The pure carbon dioxide, mixture methane + carbon dioxide at infinite dilution of supercritical carbon dioxide and water + carbon dioxide at infinite dilution of supercritical carbon dioxide. The usage of molecular simulation methods in supercritical region gave us a distinct advantage of knowing the microstructure of the systems in a qualitative and quantitative way. The Kirkwood-Buff theory, which predicts the influence of the solvent on the solute, enabled us to predict thermodynamic properties of supercritical phase and compare them with experimental values. We have examined the density effect on structure of the pure carbon dioxide and its solutions along its critical isotherm 4 K above its critical point. We focused our research and we present results for two basic sections, A. Equilibrium and transport properties, namely Volumetric properties; Average configurational energy; Isothermal compressibility; Diffusivity; and the Isochoric heat capacity B. Solution structures at infinite solutions, namely Radial distribution function; and Coordination number We discuss the outcomes based on the density inhomogeneities of the solvent and critical fluctuations, which are maximised at the critical point. We found that the addition of methane to supercritical carbon dioxide increases the volume of the solution and a cavitation is formed around it. On the hand, the addition of water gives a cluster around it in local structure and decrease the volume of solution. We report results also of the diffusion coefficients for the pure carbon dioxide and the mixtures in this study, which it shows an anomalous decrease close to the critical point of the pure carbon dioxide. It is a general conclusion for all the properties we have studied that the density dependence along the isotherm is maximised at densities close to the critical one. Further, the usage of both molecular dynamics and Monte Carlo in supercritical regions validates the extension of the techniques in the supercritical region and reveals their limitations.

Type:Thesis (Doctoral)
Title:Molecular simulation studies in the supercritical region
Open access status:An open access version is available from UCL Discovery
Language:English
UCL classification:UCL > School of BEAMS > Faculty of Engineering Science > Chemical Engineering

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