MOLECULAR DYNAMICS STUDIES OF MIXED GAS HYDRATE MELTS AND AQUEOUS INTERFACES

Abstract

Increasing global energy demand has necessitated the search for energy resources which can substitute the fast depleting conventional reserves. In the last few decades, gas hydrates have gained significant attention as a promising future energy resource. The advantages of gas hydrates as an energy resource includes their abundance in under ocean and permafrost regions and the low level of CO2 emission during the combustion of natural gas compared to other carbon based fuels. Several techniques for the extraction of natural gas from hydrate sediments have been developed. All these techniques involve the dissociation of the crystalline hydrate structure which is accompanied by the release of large amount of gas and water. The liquid phase formed during hydrate dissociation contains hydrate forming gas molecules dissolved in it at high concentrations. The evolution of dissolved gas molecules from this liquid is known to have a significant effect on the kinetics of hydrate dissociation. Earlier studies on the evolution of dissolved gas from the hydrate melt were limited to the case of only one type of gas molecule in the melt. The effect of thermodynamic hydrate inhibitors on the evolution of dissolved gas is also not well understood. The present thesis attempts to apply molecular dynamics simulations to study the process of dissolved gas evolution from aqueous solutions containing one or more types of the hydrate forming gases at conditions typical to natural gas extraction. An important physical system which is of significance to gas hydrate formation as well as atmospheric chemistry is the interface between liquid and gas. One of the most studied liquid-gas interface is the one between water and methane which is known to act as preferred sites for hydrate nucleation due to the high concentration of dissolved gas at the interface. The interaction of methane with water at their interface also has an important role in atmospheric processes such as adsorption of methane on aqueous aerosols. Despite this, the current understanding of structure and dynamics of this interface is insufficient to explain important interfacial processes such as methane dissolution. In the present thesis, molecular dynamics simulations are applied to study the structure of methane-water interface at a molecular level. The role of interfacial structure and the presence of an amphiphilic cosolvent on the adsorption and dissolution of methane at the interface is also examined. In Chapter 1 of the thesis, various methods for natural gas extraction through hydrate dissociation and the factors affecting the rate of dissociation are discussed. The chapter also reviews earlier studies on the evolution of dissolved gas molecules and its effect on hydrate i dissociation. Following this, findings from reported studies on the structure and dynamics of the liquid-gas interface are briefly reviewed. The second chapter briefly discusses the computational methods applied in the present work. The functional forms of the various interaction potentials and integrator algorithms applied in molecular dynamics simulations are discussed and a brief outline of the simulation procedure is given. In chapter 3, evolution of dissolved gas in the CH4-CO2-H2O ternary mixture is investigated. The study of CH4-CO2-H2O mixture is important since it is formed during the extraction of methane from hydrate sediments by its replacement in hydrate cages with CO2. Classical molecular dynamics simulations of the ternary mixture of varying compositions were performed which revealed that evolution of gas molecules from the mixture involves the formation of nanobubbles. The study also revealed that an increase in the concentration of CO2 enhanced the formation of bubbles in the mixture. To understand the role of CO2 in promoting bubble formation, the structure and composition of the nanobubbles formed were examined in terms of the average distribution of gas molecules. The analysis revealed that bubbles formed in the mixture are of mixed type with both gas molecules present inside them. The average distribution of gas molecules in the bubble indicated that CO2 molecules accumulate at the interface between the bubble and the surrounding liquid phase. The CO2 molecules at the interface interact with CH4 through direct contact which is energetically favorable than gas-water interactions. The value of surface tension at the bubble-water interface was calculated which revealed that the presence of CO2 reduces the surface tension thereby enhancing the stability of the interface. The greater stability of the interface decreases the critical size of the bubble nuclei leading to rapid bubble formation. The results suggest that an increase in concentration of CO2 assists the evolution of dissolved gas from the CH4-CO2-H2O mixture thereby preventing the accumulation of methane in the liquid phase. Thus, the presence of CO2 assists the decomposition of methane hydrates in the initial stages of the replacement process. Chapter 4 investigates the effect of thermodynamic hydrate inhibitors on natural gas evolution from a hydrate melt. The effect of two common hydrate inhibitors, NaCl and CH3OH on the formation of nanobubbles by dissolved gas molecules was studied. Both types of inhibitors considered are found to assist the formation of methane nanobubbles in the CH4-H2O system. NaCl promotes bubble formation by enhancing hydrophobic interaction bewteen aqueous gas molecules. Whereas, enhanced bubble formation in the presence of CH3OH is ii attributed to its amphiphilic nature. These molecules are found to accumulate at the interface between bubble and water with the methyl group oriented towards the gas phase. The presence of CH3OH at the interface makes the nanobubble more stable by reducing the excess pressure inside the bubble as well as surface tension at the interface. The evolution of dissolved gas from the CH4-CO2-H2O mixture containing hydrate inhibitor was also examined. It is observed that, for a given concentration of the inhibitor, nanobubble nucleation is more rapid in the CH4-CO2-H2O ternary system compared to that in CH4-H2O system. The nanobubble formed in the ternary system contains both CH4 and CO2 in it and the composition of the bubble is found to be influenced by the type and concentration of the inhibitor molecules present. The stability and properties of the nanobubble is related to its interaction with the surrounding liquid. A quantitative analysis of the bubble-liquid interaction revealed that a frequent exchange of gas molecules takes place between the bubble and the surrounding liquid. The frequency of this gas exchange is found to decrease with an increase in the concentration of hydrate inhibitor and also with an increase in the radius of the bubble. The observed trends in bubble-liquid interaction are explained in terms of the excess pressure inside the bubble and the solubility of gas molecule in the surrounding liquid phase. Considering the significance of methane-water interface to gas hydrates and atmospheric chemistry, the molecular level structure of this interface was investigated. The results of the study are discussed in chapter 5. Earlier studies on the adsorption of methane molecules on the water surface did not consider the effect of the inherent molecular level roughness of the surface on gas adsorption. Therefore, adsorption of methane on water surface was examined by taking into account the roughness of the surface. A quantitative analysis of roughness was performed in which the extend of roughness was expressed in terms of the amplitude of humps and wells of the surface as well as the frequency at which these are present at the surface. The simulation of methane-water interface at different pressures indicated that an increase in the pressure makes the water surface more rough in terms of amplitude of the humps and wells. The adsorption of methane on the rough water surface was analyzed by identifying the methane molecules in direct contact with the surface as well as those which are held slightly above the surface by attractive non-covalent methane-water interactions. The analysis revealed that a greater fraction of methane molecules in direct contact with the surface are present at the humps of the surface. In contrast, the density of methane molecules above the surface is higher near the wells compared to the humps. The results indicate a clear preference for methane to come in direct contact with iii the water surface at the humps rather than at the wells. This is caused by a lower density of water at the humps of the surface layer which reduces the free energy penalty associated with the formation of a cavity between water molecules which methane can occupy. In chapter 6, dissolution of methane at its interface with methanol-water liquid mixture is investigated. Methanol is a commonly used hydrate inhibitor and is also known to act as a cosolvent for methane in water. The entry of methane into the bulk liquid region of the methanol-water mixture is examined by determining the average density profile of methane along the direction perpendicular to the interface. The results indicate that molecular level surface roughness of the methanol-water liquid mixture has a role in methane dissolution with humps of the surface acting as preferred channels for the entry of methane into the bulk liquid. Analysis of surface roughness of the methanol-water mixture indicates that the surface becomes more rough with an increase in the concentration of methanol. The humps and wells at the surface of methanol rich mixtures are larger in terms of their amplitude compared to the case of mixtures with lower concentrations of methanol. The larger humps on the surface of methanol rich mixtures can assist the entry of methane into the liquid by acting as channels for methane dissolution. The results suggest that the effect of methanol on roughness of liquid surface has a role in the enhanced solubility of methane in aqueous methanol. The effect of methane on the orientation of the methanol molecules at the surface of the methanol-water mixture was also examined. The results indicate that the presence of methane significantly increases the tendency of surface methanol molecules to have their methyl group oriented towards the gas phase of the interface. The summary and future scope of the present study is provided in chapter 7.