Critical Films on Graphene Substrates

When depositing a substance onto a surface, it follows that as one adds more of the substance, the thickness of the film formed on the surface will grow. However, a particle\u27s ability to adsorb onto the surface is dependent on the interaction potential and geometry. For atomically flat two-dimensional materials such as graphene, it is predicted that the interactions between the graphene sheet and light atoms in a nearby gas are so weak that film growth may become arrested at a critical thickness. We report on molecular dynamics simulations of nitrogen near graphene to explore the existence and properties of the predicted critical film

Classical and reactive molecular dynamics: Principles and applications in combustion and energy systems

Molecular dynamics (MD) has evolved into a ubiquitous, versatile and powerful computational method for fundamental research in science branches such as biology, chemistry, biomedicine and physics over the past 60 years. Powered by rapidly advanced supercomputing technologies in recent decades, MD has entered the engineering domain as a first-principle predictive method for material properties, physicochemical processes, and even as a design tool. Such developments have far-reaching consequences, and are covered for the first time in the present paper, with a focus on MD for combustion and energy systems encompassing topics like gas/liquid/solid fuel oxidation, pyrolysis, catalytic combustion, heterogeneous combustion, electrochemistry, nanoparticle synthesis, heat transfer, phase change, and fluid mechanics. First, the theoretical framework of the MD methodology is described systemically, covering both classical and reactive MD. The emphasis is on the development of the reactive force field (ReaxFF) MD, which enables chemical reactions to be simulated within the MD framework, utilizing quantum chemistry calculations and/or experimental data for the force field training. Second, details of the numerical methods, boundary conditions, post-processing and computational costs of MD simulations are provided. This is followed by a critical review of selected applications of classical and reactive MD methods in combustion and energy systems. It is demonstrated that the ReaxFF MD has been successfully deployed to gain fundamental insights into pyrolysis and/or oxidation of gas/liquid/solid fuels, revealing detailed energy changes and chemical pathways. Moreover, the complex physico-chemical dynamic processes in catalytic reactions, soot formation, and flame synthesis of nanoparticles are made plainly visible from an atomistic perspective. Flow, heat transfer and phase change phenomena are also scrutinized by MD simulations. Unprecedented details of nanoscale processes such as droplet collision, fuel droplet evaporation, and CO2 capture and storage under subcritical and supercritical conditions are examined at the atomic level. Finally, the outlook for atomistic simulations of combustion and energy systems is discussed in the context of emerging computing platforms, machine learning and multiscale modelling

Coarse grained force field for the molecular simulation of natural gases and condensates

AbstractThe atomistically-detailed molecular modelling of petroleum fluids is challenging, amongst other aspects, due to the very diverse multicomponent and asymmetric nature of the mixtures in question. Complicating matters further, the time scales for many important processes can be much larger than the current and foreseeable capacity of modern computers running fully-atomistic models. To overcome these limitations, a coarse grained (CG) model is proposed where some of the less-important degrees of freedom are safely integrated out, leaving as key parameters the average energy levels, the molecular conformations and the range of the Mie intermolecular potentials employed as the basis of the model. The parametrization is performed by using an analytical equation of state of the statistical associating fluid theory (SAFT) family to link the potential parameters to macroscopically observed thermophysical properties. The parameters found through this top-down approach are used directly in molecular dynamics simulations of multi-component multi-phase systems. The procedure is exemplified by calculating the phase envelope of the methane–decane binary and of two synthetic light condensate mixtures. A methodology based on the discrete expansion of a mixture is used to determine the bubble points of these latter mixtures, with an excellent agreement to experimental data. The model presented is entirely predictive and an abridged table of parameters for some fluids of interest is provided

Molecular Simulation Study Of Enhanced Oil Recovery Methods In Tight Formation

The Middle Bakken Formation of the Williston Basin is a typical tight formation with the predicted primary oil recovery of less than 10%, which results in large amounts of oil remaining in the reservoir. Therefore, an effective enhanced oil recovery (EOR) method for recovering the residual oil is crucially important. To obtain the microscopic EOR mechanisms, molecular simulation methods including Monte Carlo (MC) simulations and molecular dynamics (MD) simulations were applied to study the various EOR methods, such as CO2 injection, hydrocarbon gas injection, and nanofluid flooding. A series of molecular models, including bulk systems, interfacial systems, and nanoconfined systems, were built to evaluate the potentials of the injected fluids to improve oil recovery.CO2 injection is a successful EOR technology that is being widely applied in North American oil fields. Studies have suggested CO2-based EOR is technically possible in the Middle Bakken Formation. The swelling of the crude oil/CO2 system plays a crucial role in the CO2 flooding process. Therefore, a better understanding of the effect of CO2 on crude oil swelling and viscosity reduction is critical for a successful CO2 EOR project. In this dissertation, a series of n-alkane/CO2 systems were studied by performing configurational-bias Monte Carlo (CBMC) simulations and MD simulations. The effects of alkyl chain length, pressure, and temperature on the CO2 solubility and the swelling factor were investigated. The solubility of CO2 and the swelling factor of CO2 saturated n-alkane are positively correlated to the pressure, while negatively correlated to the alkyl chain length and temperature. With more CO2 dissolved, the interaction energy between n-alkane molecules becomes less negative, which indicates the swelling of the n-alkane/CO2 system. N-alkanes with longer alkyl chain have more negative intermolecular interaction energy, and thus have a smaller swelling factor after saturating with CO2. With the increase of the CO2 mole fraction, the viscosity of the n-alkane/CO2 system is reduced. N-alkanes with longer alkyl chains have a larger viscosity reduction with increasing amounts of dissolved CO2. Besides CO2, hydrocarbon gases, like methane and ethane, can also mobilize the residual oil and enhance oil recovery. The gas solubility, volume swelling factor, oil diffusion coefficient, minimum miscibility pressure (MMP), and the oil extraction from nanoslits were then studied to compare the efficiency of different gases in the EOR process. Based on the Bakken oil composition, a molecular model of the crude oil containing different types of alkanes was built. MD simulations were carried out to study the interfacial interactions between the Bakken crude oil and the injected gases and the oil extraction from the calcite nanoslits. At various pressures and reservoir temperature, density profiles were plotted to show the distributions of different components, and the solubility of gases in crude oil was calculated. The simulation results show that all three gases hold great potential in further improving oil recovery. At constant temperature and pressure, ethane holds the highest solubility in crude oil and can induce the most pronounced oil swelling. Meanwhile, ethane can achieve the lowest MMP and the most significant oil diffusion coefficient. Without the effect of nano-confinement, ethane is most effective in mobilizing crude oil. However, CO2 is more effective in extracting oil from the nanoslits. Recent studies have also reported various types of nanoparticles (NPs) for improving oil recovery either alone or in combination with surfactants. The mechanisms of surface-modified silica (SiO2) NPs in improving oil recovery were investigated. Interfacial tensions (IFTs) of octane (C8H18)/water systems in the presence of different NPs were calculated. Quartz nanochannels were constructed to study the effect of NPs on oil flow through nanopores in rocks. Both water-wet and oil-wet surfaces were considered. Simulation results indicate that IFT reduction depends strongly on the distribution and the interfacial concentration of NPs. Surface-modified NPs with both hydrophilic and hydrophobic functional groups can reduce the IFT between oil and water. However, the IFT reduction is not significant in terms of EOR application. The alkanes/water/NPs transportation in confined nanochannels shows that the initial rock wettability affects the water flooding performance and the final oil recovery. The surface-modified NPs hold a higher capacity in detaching oil droplets from the oil-wet mineral surface regardless of their abilities to change interfacial tension. Surface modification is crucial to improve the surface properties of SiO2 NPs. The strong interactions between NPs and oil/rock lead to oil detachment and incremental oil recovery. The chemical composition of the functional groups and the surface coverage of the hydrophilic/hydrophobic functional groups should be carefully tuned to achieve the highest oil recovery rate. Molecular simulation study provides better insight into the interactions between oil components and injected fluids or mineral surfaces at the molecular level. The effect of injected fluids on the properties of the oil can be clearly explained. The application of molecular simulation methods could play an important role in interpreting experimental results and providing guidance for practical oil recovery processes in the Bakken Formation

Doctor of Philosophy

dissertationShale plays have revolutionized oil and gas production in the United States. In the last decade, many shale gas and liquid plays have been explored and developed in the US and elsewhere. Prospective shales consist of a complex organic component known as kerogen which is a precursor to oil and gas. Shales have pores with dimensions in the range of nanometers in the organic and inorganic constituents. The presence of organic matter and nanometer pores affect the thermodynamic properties of fluids in these rocks. A hypothesis has been proposed and proved through modeling and experiments to account for the influence of kerogen on thermodynamic properties of hydrocarbon fluids. Kerogen preferentially absorbs hydrocarbons and subsequently swells in volume. This splits oil in liquid-rich shale plays into two phases â€" a retained phase and a free phase, both of which remain in equilibrium. The retained and free phases together form in-situ oil; equilibrium of in-situ oil with gas was studied to investigate the effect of kerogen on saturation pressures of oils in shales. Results indicate a bubble point suppression between ~ 4150 kPa and ~ 16350 kPa from an original value of 28025 kPa for produced Eagle Ford oil. This is attributed to the presence of kerogen. This suppression depends on the type and level of maturity of the kerogen. The confinement of hydrocarbon fluids in the nanometer pores present in shales also changes the behavior of these fluids. Pore-wall â€" fluid interactions become dominant at the nano-scale and conventional equations of state(EOS) fail to include the effect of these confined state interactions. Gibbs Ensemble Monte Carlo simulations were performed in this work to investigate the thermodynamic properties of pure components and fluid mixtures in confined pores. Suppression of critical densities and critical temperature of confined decane, decaneâ€"methane, and decaneâ€"carbon-dioxide was observed from the bulk properties. This leads to changes in the saturation pressures of fluids in the confined state. Experiments on kerogen isolated from a shale and oil were performed with a differential scanning calorimeter and a thermogravimetric analyzer. These experiments complimented the modeling results and thus, verified the effect of kerogen and hydrocarbon fluid confinement observed in the models. Finally, for gas-rich shales, a carbon dioxide injection as the most effective method was evaluated for enhanced production of gas sorbed in kerogen. Molecular modeling indicates that the carbon dioxide can replace methane sorbed in the kerogen and the kerogen matrix decreases in volume during this process. The carbon dioxide shows higher retention in the kerogen than methane, indicating the viability of enhanced gas recovery and carbon dioxide sequestration

Correlating Antiagglomerant Performance with Gas Hydrate Cohesion

Although inhibiting hydrate formation in hydrocarbon–water systems is paramount in preventing pipe blockage in hydrocarbon transport systems, the molecular mechanisms responsible for antiagglomerant (AA) performance are not completely understood. To better understand why macroscopic performance is affected by apparently small changes in the AA molecular structure, we perform molecular dynamics simulations. We quantify the cohesion energy between two gas hydrate nanoparticles dispersed in liquid hydrocarbons in the presence of different AAs, and we achieve excellent agreement against experimental data obtained at high pressure using the micromechanical force apparatus. This suggests that the proposed simulation approach could provide a screening method for predicting, in silico, the performance of new molecules designed to manage hydrates in flow assurance. Our results suggest that entropy and free energy of solvation of AAs, combined in some cases with the molecular orientation at hydrate–oil interfaces, are descriptors that could be used to predict performance, should the results presented here be reproduced for other systems as well. These insights could help speed up the design of new AAs and guide future experiments

Experimental and theoretical adsorption studies in tuneable organic-inorganic materials

Adsorption processes are widely used for the storage and separation of gases in many industrial and environmental applications. The performance of the process depends strongly on the adsorbent and its interaction with the gases. Therefore, the idea of tailoring the adsorbent to the application by adapting the pore size and/or the chemical composition is very attractive. This work focuses on two groups of customizable hybrid materials: Firstly, in crystalline metal-organic frameworks (MOFs) the chemical and structural properties can be modified by changing the metal-oxide corner or the organic linker. Secondly, periodic mesoporous silica materials can be prepared with different pore sizes and geometries depending on the surfactant and its concentration and additionally modified with organic surface groups. The adsorption behaviour of the materials can be predicted by molecular simulation and thus the influence of modifications can be studied without the need of synthesising the material. For MOFs, the coordinates of the atoms can be obtained from XRD measurements. The quality of the predicted adsorption results was investigated for pure gas (methane, ethane, propane, nitrogen and carbon dioxide) and gas mixture (methane – carbon dioxide) adsorption on the metal-organic framework CuBTC. The comparison showed a good agreement between experimental and simulated results especially at low pressures. In order to create atomistic models for the mesoporous silica structures that are amorphous on the atomistic level, two existing simulation methods to model MCM-41-type materials were combined: micellar structures from coarse grained simulations that capture the phase separation in the surfactant/silica/solvent mixtures were used as input in kinetic Monte Carlo simulation that created the pore model on the atomistic level. The model created with this new methodology showed similar adsorption behaviour compared with a model created only with the kMC method using an ideal geometrical structure as micelle. The influence of modifications of the MOF structures (exchange of metal, linker length/composition and catenation) was investigated by Grand Canonical Monte Carlo simulations for hydrogen adsorption at low temperature and temperature controlled desorption. The peaks in the desorption spectra could be related to steps in the adsorption isotherms at 20 K

Doctor of Philosophy

dissertationShale resources provide a tremendous opportunity for a long-term viable energy source, but the lower hydrocarbon recovery rates are hindering the economic development of shale reservoirs. One of the main reasons for the lower hydrocarbon recovery rates is the inadequate understanding of the fate of various injected fluids and the recovered hydrocarbons during various stages of exploration and production. As Darcy's law is limited in describing the multiphase fluid transport in shale, a comprehensive simulation framework is necessary, enabling the replication of the nanometer and subnanometer pores found in organic and inorganic matrices, and the simulation of the multiphase fluid flow in these nanopores, thus improving the comprehension of the pore-scale fluid transport process in shale reservoirs. A molecular dynamics simulation-based framework is developed in present research to address the above-defined challenges. The applications of various open-source molecular modeling tools are integrated to develop molecular pore structures found in the organic and inorganic matrices. An application of the general-purpose DREIDING force field is extended to simulate the kerogen. A gas-liquid (methane and water) transport is simulated in nanopores confined in the organic and inorganic matrices, and various dynamic transport properties of fluids (subjected to confinement) are determined to gain the qualitative and the quantitative understanding of the fluid flow. The present research provides a powerful molecular dynamics simulation-based framework that will enable the development of more complex models of nanoporous shale structures and address numerous challenges encountered in hydrocarbon recovery from shale reservoirs

Models and Computational Methods Applied to Industrial Gas Separation Processes and Enhanced Oil Recovery

[eng] Two main topics are treated in this doctoral thesis from a theoretical and computational point of view: the gas capture and separation from post-combustion flue gases, and the enhanced oil recovery from oil reservoirs. The first topic evaluates the separation of CO2 using three different materials. First, several zeolites from the Faujasite family are studied with a combination of Density Functional Theory (DFT) and Monte Carlo methods. The former is employed to understand the driving mechanisms of adsorption, whereas the latter served to assess the separation of CO2 from a flue gas formed by a ternary mixture of CO2, N2 and O2. Second, the adsorption of CO2, N2 and SO2 into Mg-MOF-74 obtained through DFT calculations is presented to determine the most fundamental gas/MOF interactions. The results are then coupled to a Langmuir isotherm model to derive the macroscopic adsorption isotherms of the three gases in Mg-MOF-74. Finally, the absorption of CO2 and SO2 into three different phosphonium-based Ionic Liquids (ILs) is addressed by using the soft-SAFT equation of state and the COSMO-RS model. From the calculated adsorption/absorption isotherms several properties are obtained, such as the purity in the recovered gas, the working capacity of the materials and their selectivity to capture CO2 in the presence of other contaminant species. The main results obtained from this part of the thesis reveal that the cations of microporous materials are very strong sites of absorption for polar gases (i.e., the Na+ cations in Faujasites or the Mg2+ cations in Mg-MOF-74). This feature makes them very good candidates for CO2 capture, but they can be easily poisoned by other polar gases such as SO2. For this reason, it is highly recommended to desulphurize the flue gas before using any of these adsorbents. Similarly, ILs have higher affinity for SO2 than for CO2. However, the gas/IL interactions are significantly weaker, so they do not become poisoned by SO2. This fact implies that SO2 can be captured and separated from the flue gas by using a phosphonium-based IL. The second topic describes via Molecular Dynamics simulations the interactions of several model oils with different rocks and brines. The obtained insight can be applied in better understanding the interactions of the species present at oil reservoirs, with direct application in enhanced oil recovery processes. To that end, two wettability indicators are monitored to determine the potential recovery of the model oils. First, the oil/water interfacial tension (IFT) under different conditions of temperature, pressure and salinity (i.e., from pure water to 2.0 mol/kg of NaCl or CaCl2). And second, the oil/water/rock contact angle (CA) on calcite (10-14) and kaolinite (001) also as a function of salinity (i.e., from pure water to 2.0 mol/kg of NaCl or CaCl2). The different model oils are built with molecules of different chemical nature representing the Saturate/Aromatic/Resin/Asphaltene (SARA) fractionation model. In a final stage of the doctoral thesis the effect of non-ionic surfactants at the oil/brine IFT is also included. The main results obtained show that the most polar components of oil migrate to the oil/water interface and reduce the IFT. However, the same compounds feel attracted to the rock, who increase the CA and hamper the oil recovery. Some of these interactions are affected by the presence of salt. Specifically, if a water layer is formed between the oil and the rock in a reservoir, electrolytes can diffuse into it and attract the polar components of oil, ultimately increasing the CA. Finally, cations can be attracted to the oil/water interface due to salt/surfactant interactions. Both species interact synergistically to modify their orientation/distribution at the interface and reduce the oil/water IFT.[cat] En aquesta tesi doctoral s’han tractat dos temes principals des d’una perspectiva teòrica i computacional: la captura i separació de gasos de post-combustió, i la recuperació millorada de petroli. El primer tema avalua la separació de CO2 utilitzant tres materials diferents. Primer, s’han estudiat diverses zeolites de la família de les Faujasites amb una combinació de teoria del funcional de la densitat (TFD) i mètodes Monte Carlo per entendre els mecanismes d’adsorció separació de CO2 d’una mescla ternària que conté CO2, N2 i O2. Seguidament, s’ha presentat un estudi TFD d’adsorció de CO2, N2 i SO2 en Mg-MOF-74 per determinar les interaccions fonamentals del MOF amb cada gas. Aquesta informació s’ha acoblat a un model d’isoterma de Langmuir per tal de derivar les isotermes d’adsorció macroscòpiques dels tres gasos en Mg-MOF-74. Finalment, s’ha analitzat l’absorció de CO2 i SO2 en tres Líquids Iònics (LIs) basats en fosfoni mitjançant l’equació d’estat soft-SAFT i el model COSMO-RS. D’altra banda, el segon tema descriu les interaccions de diferents models de petroli amb roques i salmorres, via simulacions de Dinàmica Molecular. El coneixement adquirit en aquesta part de la tesi doctoral es pot aplicar directament a la recuperació millorada de petroli i per entendre millor les interaccions de les espècies presents als pous. Amb aquesta finalitat, s’han controlat dos indicadors de la mullabilitat per determinar la recuperació potencial d’aquests models de petroli. Primer la tensió interfacial (TIF) oli/aigua sota diferents condicions de temperatura, pressió i salinitat (des d’aigua pura a 2.0 mol/kg de NaCl o CaCl2). I segon, l’angle de contacte oli/aigua/roca en calcita (10-14) i caolinita (001) en funció de la salinitat (des d’aigua pura a 2.0 mol/kg de NaCl o CaCl2). Els diferents models de petroli s’han construït amb molècules de diferent naturalesa química representant el model de fraccionament Saturat/Aromàtic/Resina/Asfaltè (SARA). En una etapa final de la tesi doctoral s’ha inclòs l’efecte en la TIF induïda pels surfactants no-iònics a la interfase oli/salmorra

Development of coarse-grained force fields from a molecular based equation of state for thermodynamic and structural properties of complex fluids

In spite of the vast array of modelling techniques and force fields available, the study of the phase behaviour, structure, microstructure, and dynamics of mixtures remains a challenging task. A systematic coarse-graining (CG) methodology is employed in this thesis involving the parameterisation of force fields using a top-down approach, by effectively describing a large number of target macroscopic thermodynamic states with a rigorous molecular-based equation of state. A recent incarnation of the Statistical Associating Fluid Theory (SAFT-gamma) is used. The underlying force field is based on the Mie intermolecular potential, which is a generalised form of the Lennard-Jones potential with a variable and versatile form of the repulsive and attractive interactions. The coarse-grained force fields developed in this manner are used directly in Molecular Dynamics simulations in order to explore the dynamical, structural, and interfacial properties, which can not be directly accessed by the equation of state (unless a suitable treatment of the inhomogenous properties of the system is made). The goal of any coarse-graining procedure is to derive simple, but accurate, robust, and transferable force fields. By aiming for the simplicity, the coarse-grained models developed in our work are typically based on the three-to-one mapping, i.e., one bead containing approximately three heavy atoms, or one-to-one mapping for the small spherical molecules, with the polar, directional, and long-ranged interactions between the beads treated implicitly using the effective spherically-symmetric Mie potentials. The SAFT-gamma Mie coarse-graining methodology is exemplified for a number of fluid systems of different complexities, including pure component systems, such as: the homologous series of n-alkanes, n-perfluoroalkanes, semifluorinated alkanes, ethers and water; binary and ternary mixtures, comprising the carbon dioxide, n-alkanes, and water; and finally the aqueous mixtures of alkyl polyoxyethylene glycol non-ionic surfactants. An accurate representation of the vapour-liquid properties with both, the equation of state and molecular simulation, is obtained for the molecules of different size and chemical nature. Describing the properties of water is, however, a much more difficult task. The CG model suffers from issues associated with the transferability and representability of the various properties for different thermodynamic conditions, as a consequence of the aggressive averaging of the strongly directional and polar forces into an effective spherically symmetrical potential. It has been shown that an isotropic single-site CG model based on a spherically symmetrical potential cannot capture all of the thermodynamic properties of water simultaneously (the issue of representability). Two different CG models of water are proposed: the first is designed to accurately reproduce the saturation liquid density and vapour pressure, and the second to capture the saturation liquid density and surface tension with high precision. Both models benefit from an accurate parameterisation of temperature dependence following the target properties over the entire temperature range of the fluid. An additional model is developed based on the two-to-one mapping, enabling more efficient large scale simulations in, for example, biomolecular systems. The models of the binary mixtures are developed by using the corresponding pure component models with an additional adjustable parameter to account for the unlike interactions; the latter are obtained by considering appropriate properties of the mixtures such as the fluid-phase equilibria or the thermodynamic properties of mixing. The unlike interactions are shown to be transferable for a quantitative description of the phase behaviour over a wide range of conditions and for the systems of related components. We are able to obtain an accurate prediction of the azeotropic point, critical loci, tree phase line, global density, and the shape of phase envelopes for studied mixtures. The quality of predictions is found comparable to the results from the atomistic models and other equations of state. The aqueous mixtures of alkyl polyoxyethylene glycol non-ionic surfactants are a key final goal of the research presented in this thesis. The CG models of the surfactants are developed within the SAFT-gamma group-contribution framework, where each functional group is derived from an accurate representation of the corresponding chemical moiety. By capturing a delicate interplay of the repulsive and attractive intermolecular interactions and obtaining the right balance between energetic and entropic effects, the various phase morphologies at ambient conditions can be reproduced in agreement with the experimental findings over the entire concentration range. The force fields developed in the current work allow for a prediction of key structural and interfacial properties. The Molecular Dynamics simulations reveal the spontaneous formation of micelles at low surfactant concentrations and a self-assembly into a bilayer at high surfactant concentrations. The aggregation numbers, the critical micelle concentration, area per molecule, the surface excess properties, and bilayer thickness are found in very good agreement with experimental data. This is very encouraging considering that only macroscopic thermophysical properties are used to develop the underlying force fields that describe the fine interactions between the molecules in the system. Despite the simplicity, coarse-grained force fields are shown to be robust and transferable; they can be applied to predict the properties which were not used in the original parameterisation procedure, with an accuracy comparable to the more sophisticated and computationally demanding models.Open Acces