Confinement Effects on Non-Reactive and Reactive Transport Processes: Insights from Molecular Dynamics Simulations

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

Adsorption of Light Alkanes on the Surface of Substrates with Varying Symmetry and Composition

Adsorption plays an integral role in a variety of fundamentally and technologically important processes such as lubrication, gas separation and purification, wetting behavior, energy storage, heterogenous catalysis, biologically inspired materials, and the theory of phase transitions. As a result, adsorption phenomena are extensively studied in chemistry, physics, and biology each for uniquely different reasons. The homologous series of normal alkanes represent a class of organic molecules that are important in the fuel industry. From a fundamental perspective, the series of normal alkanes provide a route whereby physical and chemical properties relevant to adsorption can be examined with only subtle changes in molecular size and length. The alkanes also exhibit a well-known odd-even effect in some condensed phase physical properties. In the current study, the physical adsorption properties of the normal alkanes (methane-decane) on MgO, graphite, and boron nitride were investigated using volumetric adsorption isotherms and molecular dynamics simulations. This portion of the study focuses on determining the thermodynamics of adsorption as well as predicting the adsorption structures and dynamics. As a secondary study, the chemical adsorption of ethanol was examined on the surface of transition-phase aluminas using volumetric adsorption, temperature-programmed desorption, and inelastic neutron scattering. The purpose of this work was to observe the surface-catalyzed reaction of chemically bound ethanol with Lewis and Brø[oe]nsted acid sites present on the aluminas in-situ. The results of the projects described have significant implications in the design of new materials for gas separation and purification as well as heterogenous catalysis

Gas adsorption in the MIL-53(AI) metal organic framework. Experiments and molecular simulation

Dissertação para obtenção do Grau de Doutor em Engenharia QuímicaFCT - PhD Fellowship at Universidade Nova de Lisboa, Department of Chemistry (bolsa N SFRH/BD/45477/2008); FCT Program, project PTDC/AAC-AMB/108849/2008; NANO_GUARD, Project N°269138; Programme “PEOPLE” – Call ID “FP7-PEOPLE-2010-IRSES

Experimental research on turbulent reacting flows using gaseous and liquid fuels

An investigation into turbulent reacting flows in an opposed jet geometry and a sudden expansion duct has been performed. For the opposed jet geometry, measurements of the velocity and reaction progress variable were obtained in lean premixed flames. Both velocity and scalar measurements were taken using PIV (Particle Image Velocimetry). Three gaseous fuels (methane, propane and ethylene) and three liquid fuels (JP-10, cyclopentane and cyclopentene) were considered for a range of equivalence ratios. The broad range of fuels enabled an investigation of the effect of different fuel reactivities on the velocity field and flame location and also allowed the effect of the Lewis number on flame extinction to be investigated. Preliminary work included isothermal measurements of the flow between and inside the nozzles. The use of fractal grids inside the nozzle increased turbulence intensities at the nozzle exit by 100% and turbulent Reynolds numbers between 50 - 220 were achieved. Velocity and normal stress components were measured with attention focused on the inlet boundary, along the burner centreline and the stagnation plane. A circular duct, incorporating a sudden expansion step, was also used to investigate the effect of swirl on pressure oscillations within the duct, the lean flammability limits and the NOx emissions. Measurements were performed for stratified flow conditions using methane as a fuel. The results show that excessive swirl leads to an increase in local strain in the vicinity of the expansion step and makes the flame more prone to local extinction. Moderate swirl was found to lower the amplitudes of the pressure oscillations close to global extinction and also to decrease the lean extinction limit of the stratified flow conditions. However, it did not decrease the overall equivalence ratio of flows with a richer core and a leaner annulus. Flows with only air in the core flow led to an overall equivalence ratio as lean as 0.3 for methane compared with 0.6 for the uniform flow. Stratification with a fuel rich core flow and a leaner annular flow led to an increase in NOx emissions due to locally increased temperatures. The addition of moderate swirl enhanced mixing of the annular and the core flows, which resulted in a more uniform fuel distribution close to the step and a reduction in NOx-levels up to 50%

Hydrate Phase Transition-Risk, Energy Potential and CO2 Storage Possibilities

Natural gas hydrate (NGH) can cause crucial flow assurance problems to the oil and gas industry. It is being considered as a potential vast energy resource for the world in the future. It could also potentially provide a long-term offshore storage possibility for carbon dioxide. Therefore, the risk of hydrate formation during processing and pipeline transport of natural gas and CO2, thermodynamics and kinetics of hydrate formation, and simultaneous CH4 production from in-situ hydrate and CO2 long-term offshore storage in form of CO2 hydrate are important research concerns. The main scientific method used in this project is classical thermodynamics based on thermodynamic properties calculated using methods in Quantum Mechanics and Classical Mechanics. Classical thermodynamics was used together with residual thermodynamics description for every phase; this includes the hydrate phase, to analyse different routes to hydrate formation between hydrate formers (or guest molecules) and water. NGHs are formed from water and natural gas at high pressures and low temperatures conditions under the constraints of mass and heat transport. The problem is that natural gas is usually produced together with water and operations are usually at elevated pressures and low temperatures. Current industrial approach for evaluating the risk of hydrate formation is based on liquid water condensing out of the bulk gas at dewpoint and at a specific pressure-temperature (P-T) condition. In this method, the maximum allowable water content will be kept below the projected dew-point mole-fractions during transport, considering the operational P-T conditions. However, a previous study in our research group suggested that solid surface, particularly rust (Hematite) is another precursor to hydrate formation; rust provides another route for liquid water to drop out through the mechanism of adsorption. And pipelines are generally rusty before they are mounted in place for operations. The two approaches have been applied to study the risk of water dropping out from natural gas from different real gas fields. The approach of adsorption of water onto Hematite (rusty surfaces) completely dominates. The dew-point method over-estimates the safe limit (maximum mole-fraction) of water that should be permitted to flow with bulk gas about 18 – 20 times greater than when the effect of hematite is considered, depending on the specific gas composition. That suggests that hydrate may still form when we base our hydrate risk analysis on dew-point technique. The presence of higher hydrocarbon (C2+) hydrate formers causes a decrease in allowable water content with increasing concentration of ethane, propane and isobutane for the temperature range of 273 – 280 K. As their concentrations increase in the bulk gas, these C2+ act to draw down the water tolerance of the gas mixture to a point where they completely dominate or dictate the trends. For the inorganic components, CO2 has little or no significant impact on the allowable upper-limit of water when its concentration increases. While the presence of H2S causes a consideration reduction in water tolerance of the system as its concentration in the mixture increases. The presence of 1 mol% of H2S in the bulk gas may cause about 1 % reduction in water tolerance. The reduction in maximum content of water could be up to about 2 – 3 % and up to about 4 – 5 % if the concentration increases to 5 mol% and 10 mol% respectively. It is not appropriate to interpret hydrate stability entirely based on equilibrium P–T curves as often done in literature. The hydrate stability curve of CO2 hydrate has lower pressures (thus more stable) compared to that of CH4 hydrate but only to a certain temperature. That is the quadruple-point were phase-split occurs causing the pressures of CO2 hydrate going above that of CH4 hydrate due to the increase in density caused by the CO2 liquid phase. A free energy analysis revealed that CO2 hydrate has lower free energy across the entire temperature range, thus more stable at all the temperatures. Therefore, hydrate stability should rather be based on free energy analysis since in real situations hydrate cannot reach equilibrium. Consequently, the most stable hydrate is the hydrate with the minimum free energy. The hydrate with the least or most negative free energy will first form under constraints of mass and heat transport, then followed by the subsequent most stable hydrate. Among the hydrocarbon guest molecules studied, the most stable hydrate is hydrate of isobutane, followed by that of propane, and then by ethane. Induction times are sometimes mistaken as hydrate nucleation times, which is why some works report nucleation times of hours. Hydrate formation is a nano-scale process, and the hydrate nucleation times computed for both heterogeneous and homogeneous hydrate formation in this project are in nano-seconds. The long times experienced before hydrates are detected are caused by mass transport limitations due to the initial thin hydrate film formed at the interface between water and the hydrate former interface. Another misunderstanding about hydrate nucleation is that only one uniform-phase hydrate is formed from either a single guest or a multicomponent mixtures of hydrate formers. Based on the combined first and second laws of thermodynamics, nucleation will commence with the most stable hydrates, under the constraints of heat and mass transport. Nucleation can happen via different routes: hydrate formation will originate at the interface between the guest molecule phase and water. A range of hydrates with different compositions of the original hydrate former(s), different densities and different free energies will form from aqueous solution (dissolve hydrate formers). Theoretically, hydrate can also nucleate from water dissolved in the guest molecules phase. Such hydrate cannot be stable because of the little mass of water that will dissolved in the guest molecule phase as well as limitation of heat transport, especially in the case of hydrocarbon guests like methane which is a poor heat conductor. The thermodynamics of simultaneous natural gas production from in-situ CH4 hydrate and CO2 long-term offshore storage was studied. Two processes where studied: mixing of nitrogen with the CO2 and injecting the mixture into the hydrate reservoir and the implication of the enthalpies of hydrate phase transitions. The study indicated that the proportion of CO2 needed in the CO2/N2 mixture is only about 5 – 12 % without H2S in the gas stream. While it is about 4 – 5 % and 2 – 3 % with the presence of 0.5 % and 1 % of H2S respectively. Virtually, direct solid-state CO2–CH4 swap will be extremely kinetically restricted, and it is not significant. Enthalpy changes of hydrate phase transition in literature obtained from experiment, Clausius-Clapeyron and Clapeyron models are limited and often lack some vital information needed for proper understanding and interpretation. Information on thermodynamic properties such as pressure, temperature (or both), hydrate composition, and hydration number are often missing. The equation of state utilised is also not stated in certain literature. Several experimental data also lack any measured filling fractions, and frequently, they apply a constant value which suggests that the values may be merely guessed. In addition, older data based on Clapeyron equation lack appropriate volume corrections. The calculations of both Clausius-Clapeyron and Clapeyron equations are based on hydrate equilibrium data of pressure and temperature from experiments or calculated data. But hydrate formation is a non-equilibrium process. Information about superheating above the hydrate equilibrium conditions to totally dissociate the gas hydrate to liquid water and gas is normally lacking. The values vary considerably in such a way that some of them decided to base their results on average values over a range of temperatures. For example, Gupta et al. (2008) conducted a study with experiment, Clausius-Clapeyron and Clapeyron equations but all the results varied substantially. We therefore propose a method based on residual thermodynamics which does not have the limitations of the current methods. We do not expect much agreement of our results with a lot of the literature, firstly, because of the limitations of the other methods, especially, the simplicity of both the Clapeyron and Clausius-Clapeyron equations. Secondly, the remarkable disagreement among current data reported in literature. The residual thermodynamics scheme used in this project is based on the unique and straight forward thermodynamic relationship between change in free energy and enthalpy change, with thermodynamic properties evaluated from residual thermodynamics. Such properties are change in free energy as the thermodynamic driving force in kinetic theories, equilibrium curves, and enthalpy changes of hydrate phase transition. With residual thermodynamics, real gas behaviour taking into account thermodynamic deviations from ideal gas behaviour can be evaluated. The results of enthalpy changes of carbon dioxide hydrate phase transitions using residual thermodynamics in this project are around 10 – 11 kJ/mol guest molecule greater than the ones of methane hydrate phase transition for 273 – 280 K range of temperatures. Calculations based on kJ/mol hydrate within the same temperature range gave 0.5 – 0.6 kJ/mol hydrate. Anderson’s results using Clapeyron equation are a little close to the results obtained in this work, precisely 10 kJ/mol and 7 kJ/mol guest molecule at 274 K and at 278 K respectively. While Kang et al. (2001) in their experiment put this difference at 8.4 kJ/mol guest molecule at 273.65 K. However, in replacement of in-situ CH4 hydrate with CO2, it is not the temperature-pressure curve that is most essential, but what is most important is the difference in free energies of both hydrates, CH4 hydrate and CO2 hydrate, and the enthalpies of CO2 hydrate formation relative to the enthalpies of CH4 hydrate dissociation. The free energy of CO2 hydrate is around 1.8 – 2.0 kJ/mol more negative or lower than the free energy of CH4 hydrate within a temperature range of 273.15 – 283.15 K (0 – 10 °C). That confirms that hydrate of CO2 is more stable thermodynamically than hydrate of CH4. It is pertinent to state that this proposition is still under investigation, and it is still under development. In addition, there are constraints that are also under study. Hydrate formation at the interface between CO2 gas and liquid water is very rapid, forming a hydrate film which will quickly block the pore spaces thereby limiting further CO2 supply. Studies also need to be done on finding the most efficient and effective way to reduce the thermodynamic driving force, either by using any thermodynamic inhibitor or other substances.Doktorgradsavhandlin

Modeling adsorption in metal-organic frameworks with open metal sites : propane/propylene separations

We present a new approach for modeling adsorption in metal-organic frameworks (MOFs) with unsaturated metal centers and apply it to the challenging propane/propylene separation in copper(II) benzene-1,3,5-tricarboxylate (CuBTC). We obtain information about the specific interactions between olefins and the open metal sites of the MOP using quantum mechanical density functional theory. A proper consideration of all the relevant contributions to the adsorption energy enables us to extract the component that is due to specific attractive interactions between the pi-orbitals of the alkene and the coordinatively unsaturated metal. This component is fitted using a combination of a Morse potential and a power law function and is then included into classical grand canonical Monte Carlo simulations of adsorption. Using this modified potential model, together with a standard Lennard-Jones model, we are able to predict the adsorption of not only propane (where no specific interactions are present), but also of propylene (where specific interactions are dominant). Binary adsorption isotherms for this mixture are in reasonable agreement with ideal adsorbed solution theory predictions. We compare our approach with previous attempts to predict adsorption in MOFs with open metal sites and suggest possible future routes for improving our model

Computational characterization and prediction of metal-organic framework properties

In this introductory review, we give an overview of the computational chemistry methods commonly used in the field of metal-organic frameworks (MOFs), to describe or predict the structures themselves and characterize their various properties, either at the quantum chemical level or through classical molecular simulation. We discuss the methods for the prediction of crystal structures, geometrical properties and large-scale screening of hypothetical MOFs, as well as their thermal and mechanical properties. A separate section deals with the simulation of adsorption of fluids and fluid mixtures in MOFs