Le, Tran Thi Bao; (2020) Confinement Effects on Non-Reactive and Reactive Transport Processes: Insights from Molecular Dynamics Simulations. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Exploring the thermodynamic, structural and transport properties, coupled with the reactivity of complex geo-fluids in porous systems is vital in geochemistry, and it also has repercussions in a variety of fields, most importantly the manufacturing of chemicals in the industry. Experimental and computational studies can shed light on the behaviour of fluids in confinement, thereby providing insights for industrial applications in various areas such as catalysis, gas recovery, separations, and adsorption. This thesis seeks to obtain some fundamental understanding of the behaviour of fluids confined in narrow pores as well as the role of pores in reactive-transport processes by implementing the atomistic molecular dynamics (MD) simulation techniques. In collaboration with experimentalists, validation has been achieved for selected systems. The systems were simulated as confined within a realistic cylindrical pore of diameter ~16 Å carved out of amorphous silica. A series of MD simulations implementing classical force fields were conducted to examine the effect of bulk pressure and water loading on the mobility of propane confined within cylindrical silica pores. The transport properties of propane were found to depend on pressure, as well as on the amount of water present. At high H2O loading, propane transport is hindered by “molecular bridges” formed by water molecules. The results are in quantitative agreement with neutron scattering data conducted for propane-water systems confined in MCM-41–type materials. To investigate the effect of narrow pores on the possible abiotic synthesis of methane in sub-surface conditions, MD simulations implementing the reactive force field (ReaxFF) formalism were performed. Although the ReaxFF force fields were successfully parameterized to describe dynamics of complex reactive chemical systems, the simulation results reveal that they can also be able to reliably predict bulk properties of nonreactive pure fluids (CH4, CO2, H2O, and H2). However, the agreement with both simulations implementing classical force fields and experiments depends strongly on fluids and thermodynamics conditions considered. When ReaxFF molecular dynamics simulations were conducted for CO2 in the presence of excess H2 within the amorphous silica nanopores, no CH4 was obtained at the conditions considered; however, CO was found to be a stable product, suggesting that the silica pore surface facilitates the partial reduction of CO2 to CO. Because the results could be important for CCUS applications, we investigated the wetting properties of calcite in the presence of water and CO2, at various pressures and salt content. Comparison with experiments suggests that much fundamental research is still needed to design safe and reliable geological storage repositories.

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