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OS6A-3:CONSISTENCY IN VAN DER WAALS AND PLATTEUW THEORY: ROLE OF FLEXIBILITY OF WATER LATTICE
发布时间:2014-07-28
Srikanth RAVIPATI1 and Sudeep N. PUNNATHANAM2
Department of Chemical Engineering, Indian Institute of Science, India
Thermodynamics of clathrate hydrates has been of great interest because of their potential applications in gas storage, separation of gas mixtures, energy sector etc. Not only the applications, but also problems posed by hydrates, like plugging oil/gas pipe lines and global warming because of dissociation of methane hydrates (clathrate gun hypothesis) adds to the importance of study of clathrate hydrates. The theoretical understanding of hydrates is based on statistical thermodynamic theory proposed by van der Waals and Platteuw (vdWP theory). Similar to Langmuir adsorption model, vdWP theory treats cavities in the hydrate as adsorption sites for the guest molecule. Langmuir constant, is an important quantity in vdWP theory, which describes the interaction of guest molecule in a particular cavity with the water molecules in the hydrate. The reliability of phase equilibrium predictions using vdWP theory are greatly influenced by accuracy of Langmuir constant calculation. Recently [1], we have improved vdWP theory by incorporating flexibility of water lattice, to exactly calculate the Langmuir constant, for a given molecular model. Consistent vdWP theory requires only unique set of formation properties in phase equilibrium calculations for variety of guest molecules which do not distort the hydrate lattice significantly. Our ongoing research shows that inclusion of water lattice flexibility and appropriate water-guest interaction parameters lead to consistency in vdWP theory. But, earlier study was done for only rigid molecules. Here, we extend the same methodology for flexible molecules like propane, n-butane etc. linear alkanes. In earlier studies, flexible molecules have been treated as single site molecules in Langmuir constant calculation. Here, we use configurational bias Monte Carlo (CBMC) method to incorporate the flexibility of guest molecules. In this study, we use TIP4P/2005 model for water and united atom model for guest molecules. We compute Langmuir constants and guest-guest interactions for guest molecules in empty hydrate lattice and compare the predictions of occupancy using these values with grand canonical Monte Carlo simulation occupancy values. We also compute hydrate phase equilibrium and compare with the existing experimental data.
Department of Chemical Engineering, Indian Institute of Science, India
Thermodynamics of clathrate hydrates has been of great interest because of their potential applications in gas storage, separation of gas mixtures, energy sector etc. Not only the applications, but also problems posed by hydrates, like plugging oil/gas pipe lines and global warming because of dissociation of methane hydrates (clathrate gun hypothesis) adds to the importance of study of clathrate hydrates. The theoretical understanding of hydrates is based on statistical thermodynamic theory proposed by van der Waals and Platteuw (vdWP theory). Similar to Langmuir adsorption model, vdWP theory treats cavities in the hydrate as adsorption sites for the guest molecule. Langmuir constant, is an important quantity in vdWP theory, which describes the interaction of guest molecule in a particular cavity with the water molecules in the hydrate. The reliability of phase equilibrium predictions using vdWP theory are greatly influenced by accuracy of Langmuir constant calculation. Recently [1], we have improved vdWP theory by incorporating flexibility of water lattice, to exactly calculate the Langmuir constant, for a given molecular model. Consistent vdWP theory requires only unique set of formation properties in phase equilibrium calculations for variety of guest molecules which do not distort the hydrate lattice significantly. Our ongoing research shows that inclusion of water lattice flexibility and appropriate water-guest interaction parameters lead to consistency in vdWP theory. But, earlier study was done for only rigid molecules. Here, we extend the same methodology for flexible molecules like propane, n-butane etc. linear alkanes. In earlier studies, flexible molecules have been treated as single site molecules in Langmuir constant calculation. Here, we use configurational bias Monte Carlo (CBMC) method to incorporate the flexibility of guest molecules. In this study, we use TIP4P/2005 model for water and united atom model for guest molecules. We compute Langmuir constants and guest-guest interactions for guest molecules in empty hydrate lattice and compare the predictions of occupancy using these values with grand canonical Monte Carlo simulation occupancy values. We also compute hydrate phase equilibrium and compare with the existing experimental data.
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