Modeling investigation of gas hydrate decomposition: thermodynamic approach and molecular dynamic simulations

Kondori, Javad (2019) Modeling investigation of gas hydrate decomposition: thermodynamic approach and molecular dynamic simulations. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (9MB)

Abstract

In the last few decades, there has been a great interest in the hydrate reservoirs for energy storage and source purposes. It has been proven that hydrates can contribute to ocean carbon cycling, global climate change, and coastal sediment stability. The permafrost and offshore environments contain enormous quantities of methane in the form of gas hydrates. In addition, the natural gas has been recently produced worldwide including in Alaska, Siberia, Japan, and North West Territories of Canada. However, the gas hydrates formation may lead to various forms of blockages in oil/gas production and transportation processes, resulting in high capital and operating costs. Detailed experimental and modeling investigations of hydrate formation and decomposition can assist to better understand the mechanisms involved in gas production from hydrates. Thus, it is important to determine the equilibrium hydrate-forming conditions so that a systematic parametric sensitivity analysis is conducted to identify the vital process and thermodynamic parameters affecting this occurrence. This project focuses on the hydrate formation/dissociation conditions where equations of state and molecular dynamic (MD) simulations are used. Giving further information, this study provides a reliable model to determine the gas hydrate formation and decomposition conditions of pure, binary, and ternary systems of hydrate gases where the van der Waals Platteuw model is utilized by combining with extended UNIQUAC model and PC-SAFT equation of state. In addition, MD simulations are conducted to investigate the microscopic mechanisms/phenomena and intermolecular forces involved in gas (pure and mixture) hydrate decomposition, where the molecular interactions, structures, and behaviours of hydrate systems need to be appropriately explored. Through a systematic design of simulation runs, the impacts of temperature, pressure, cage occupancy, and inhibitors on the hydrate dissociation are studied. Furthermore, the diffusion coefficient, density, and heat capacity of gas hydrates with different structures and compositions of methane, carbon dioxide, propane, and isobutane are determined through employing MD strategy. A very good agreement is noticed between the modeling results and the experimental data so that the value of AADT% for PC-SAFT equation of state is lower, compared to the previous EOS/thermodynamic models. The binary interaction parameters for different binary components are investigated by using experimental hydrate data, leading to better outcome compared with results obtained through fitting the VLE data. The trend of the heat capacity and density of methane hydrate obtained from the MD simulations shows a good match with the real data. The hydrate decomposition is not achieved at the equilibrium temperature at 100% cage occupancy; however, the decomposition of the methane hydrate lattice is observed when the cage occupancy reduces from 100% to 87.5% or 75% because of low stability and high diffusion coefficient of the methane molecules at low cage occupancies where the temperature and pressure are constant. The lattice parameter for the methane/water and methane/isobutane systems is calculated at a variety of pressures and temperatures. A good agreement between the experimental data and simulation results is noticed. The relative importance of inhibitors in terms of gas hydrate decomposition duration is assessed. Based on this criterion, the inhibitors are ordered as follows: methanol > ethanol >glycerol. The physical properties such as density and lattice parameter for different compositions of methane + carbon dioxide are obtained which are in agreement with those determined by experimental and theoretical techniques. According to the MD results, the structure with methane (25%) + carbon dioxide (75%) composition is almost stable under 300 K at 5 MPa; it means the best configuration to have a stable structure is when carbon dioxide and methane molecules are in large and small cavities, respectively. MD technique is used to investigate the bubble formation and evolution of carbon dioxide and methane after dissociation. Analysing the outcome of the present and previous works, the current study provides new reliable/useful information and data on the thermodynamic behaviours and molecular level of the hydrate dissociation process. It is expected that such a research investigation offers effective tips/guidelines to deal with hydrate formation and dissociation in terms of utilization, prevention, and processing.

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