Molecular modeling of hydrate-clathrates via ab initio, cell potential, and dynamic methods

• Main Author: Anderson, Brian, Ph. D. Massachusetts Institute of Technology
• Other Authors: Jefferson Tester and Bernhardt Trout., Massachusetts Institute of Technology. Dept. of Chemical Engineering.
• Format: Thesis
• Language: English
• Published: Massachusetts Institute of Technology 2005
• Subjects: Chemical Engineering; Langmuir; Methane hydrate
• Online Access: http://hdl.handle.net/1721.1/33704; http://dspace.mit.edu/handle/1721.1/33704

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2005. Includes bibliographical references. High level ab initio quantum mechanical calculations were used to determine the intermolecular potential energy surface between argon and water, corrected for many- body interactions, to predict monovariant and invariant phase equilibria for the argon hydrate and mixed methane-argon hydrate systems. A consistent set of reference parameters for the van der Waals and Platteeuw model, . and ., were developed for Structure II hydrates and are not dependent on any fitted parameters. Our previous methane-water ab initio energy surface has been recast onto a site-site potential model that predicts guest occupancy experiments with improved accuracy compared to previous studies. This methane-water potential is verified via ab initio many-body calculations and thus should be generally applicable to dense methane-water systems. New reference parameters, . and ., for Structure I hydrates using the van der Waals and Platteeuw model were also determined. Equilibrium predictions with an average absolute deviation of 3.4% for the mixed hydrate of argon and methane were made. These accurate predictions of the mixed hydrate system provide an independent test of the accuracy of the intermolecular potentials. (cont.) Finally, for the mixed argon-methane hydrate, conditions for structural changes from the Structure I hydrate of methane to the Structure II hydrate of argon were predicted and await experimental confirmation. We present the application of a mathematical method reported earlier' by which the van der Waals-Platteeuw statistical mechanical model with the Lennard-Jones and Devonshire approximation can be posed as an integral equation with the unknown function being the intermolecular potential between the guest molecules and the host molecules. This method allows us to solve for the potential directly for hydrates for which the Langmuir constants are computed, either from experimental data ...