Study of Phase Equilibria and Interfacial Properties of Systems Containing Clathrate Hydrates Using Molecular Simulations

Abstract

Gas hydrates, also known as clathrate hydrates are nonstoichiometric and crystalline solids composed of water and gas molecules. If the gas molecules are natural gas components, such as methane, ethane, propane, etc., then they are called Natural Gas Hydrates. They typically form under the conditions of high pressure and low temperature. Gas hydrates consist of a framework made up of water molecules. Each water molecule is bonded to four other water molecules via hydrogen bonds. The framework contains cavities that are occupied by the gas molecules. The water molecules are considered as the host and the gas molecules as guests. The guests and water molecules interact via van der Waals interaction forces which stabilize the hydrate. Gas hydrates are considered to be a valuable source of energy for future generations. It has been estimated that the total amount of energy available from the natural gases trapped in the form of gas hydrates exceeds that available from all the conventional fossil fuels combined. The extraction of methane from gas hydrates is expected to play an important role in future global energy supply. Recently, storing and transporting of gases like hydrogen in the form of gas hydrates has been seen as a promising alternative method due to the advantages like safety at relatively low pressures. Historically, gas hydrates have attracted wide attention in the scienti fic community due to their adverse effects in the oil and gas industry. Gas hydrates can form in deep sea natural gas transmission pipelines during natural gas production and transportation processes leading to blockage of pipelines and disruption of operations. These problems/applications demonstrate a need for development of methods/theories for accurate prediction of gas hydrate phase equilibria and strategies/materials for ow assurance. The current theoretical understanding of clathrate hydrates is based on the van der Waals and Plattew (vdWP) theory developed using statistical thermodynamics approach. Although vdWP theory is widely used to predict the phase equilibrium of gas hydrates, it is known to su er from few drawbacks. In this thesis, I address these shortcomings and present a robust thermodynamic theory for gas hydrates. The predictions of the theory are in close agreement with experimental data. I also present a method to compute phase equilibrium of semi-clathrate hydrates using molecular simulations. The results from these calculations are used to evaluate a suitable force eld for these complex materials. The problem of flow assurance is studied using molecular simulations of anti-agglomerants. The molecular simulations reveal the molecular mechanisms involved in the action of the anti-agglomerants. I present a metric for evaluating the performance of the anti-agglomerants.