Modeling of hydrogen hydrates in the presence of promoters
Item
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Title
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Modeling of hydrogen hydrates in the presence of promoters
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Identifier
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d_2009_2013:d5ca5edd834d:10410
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identifier
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10371
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Creator
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Yedlapalli, Prasad N.,
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Contributor
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Jae W. Lee
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Date
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2009
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Language
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English
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Publisher
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City University of New York.
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Subject
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Chemical engineering | Energy | Ab initio calculations | dissociation pressures | Double gas hydrates | Excess Gibbs potential model | hydrogen storage | Langmuir constants
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Abstract
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For an economical and safe supply of hydrogen, hydrogen (H2) should be stored at reasonable pressures (<100 atm) and ambient temperatures. The research is aimed at understanding and developing one such storage medium: Gas hydrates.;Gas hydrates are crystalline compounds, also known as clathrates. The H2 molecules are entrapped in a cage-like structure (cavity) of hydrogen-bonded water molecules. Their general formula is Mn(H 2O)p where M: Gas, n: number of gas molecules, p: number of water molecules. The design of gas hydrate storage systems requires accurate prediction of how equilibrium dissociation pressures vary with temperatures.;The dissociation pressures are generally predicted using van der Waals and Platteeuw theory. However, it is only applicable to gas hydrates with singly occupied cavities and cannot deal with the multiple occupancy of H 2 molecules in a single cavity. A unique feature of H2 hydrates is that they are stable when more than one molecule (either two or four) is encaged in the same water cavity. To overcome this multiple occupancy problem, it is assumed that bi-hydrogen and tetra-hydrogen molecule clusters are fictitious single molecules. The distance parameters of these clusters are obtained from DFT calculations. Ab initio calculations are performed for determining binary interaction potentials between one H2 cluster and one water molecule for various inter molecular distances and various spatial cluster configurations using GAMESS. The interaction energies are fit to the Exp 6 potential model. Then, energy parameters in the Exp 6 are used to determine Langmuir constants of hydrogen clusters in the cavities that will be used to determine the chemical potential of the hydrate phase. At equilibrium, the chemical potential of water in the liquid/ice phase will be equal to that of water in the hydrate phase. Using a modified Holder-Lee model, the equilibrium pressures are determined with respect to various temperatures.;The double hydrates of H2 and other promoters such as THF have lower formation pressures although they are formed with lower H2 storage densities. We have performed rigorous calculations to predict the dissociation pressures of binary and multi-component hydrates by proposing an excess Gibbs potential model. Finally, we investigate the stable occupancies of hydrogen molecules in the small cavity using whole double-cavity simulations. The single occupancy is as favorable as the double occupancy in the small cavity of sII hydrates.
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Type
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dissertation
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Source
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2009_2013.csv
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degree
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Ph.D.
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Program
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Engineering
