Development and Application of Simulation Methods for Clathrate Hydrates Phase Equilibria

Colorado School Of Mines, Golden CO

Investigators

Abstract

0933856 Sum Clathrate hydrates are non-stoichiometric solids (generally hydrates of methane) with intriguing molecular structures consisting of water cavities enclosing small non-polar molecules. Accurate and reliable predictions of hydrate phase equilibria require knowledge of the hydrate composition, a quantity that is not easily accessible, but is greatly needed given the increasing importance of gas hydrates in the energy resource landscape. To date, much of the knowledge on hydrate phase equilibria derives from predictions based on the solid-solution theory by van der Waals and Platteeuw, which albeit practical, has several fundamental shortcomings. This proposal undertakes a fundamental study to investigate the molecular thermodynamics of clathrate hydrates in terms of their composition. The specific objectives for this project are to develop a rigorous computational methodology to determine the composition and distribution of guests in hydrate cavities and to then apply it in the prediction of three-phase (Lw-H-V) equilibria of clathrate hydrates. Intellectual Merit. To achieve the stated objectives, this study will develop and implement computational methods capable of determining solid phase equilibria. To accomplish this, two main tasks will form the core of the study: i) Develop and implement a novel algorithm for solid-phase Gibbs ensemble Monte Carlo with helical boundary conditions to rigorously capture solid phase equilibria, initially applied to model systems and water/ice, for the eventual application to clathrate hydrates, and ii) Perform GEMC and classical thermodynamic integration (TI) simulations, in a parallel effort to determine the composition and distribution of one or more guests in clathrate hydrates, including at conditions along the three-phase (Lw-H-V) equilibrium. Although laborious, the parallel effort with the thermodynamic integration method will validate and extend the applicability of the method. The proposed work is novel and transformative in that: i) the methodology overcomes severe sampling inefficiency that heretofore prohibited direct solid phase simulations using Gibbs ensemble Monte Carlo, and ii) it will be the first to directly evaluate, either through simulation or experiment, the absolute equilibrium composition and distribution of guests in the hydrate cages. If successful, this study will greatly expand the scope of practical simulations to the many areas that involve solid phase equilibria, and significantly advance the underlying science of clathrate hydrates phase equilibria underpinning applications such as their increasing role as a prominent energy resource. Broader Impact. This study will develop a framework toward more accurate and reliable methane hydrate phase equilibria predictions, which are routinely needed in industry, and consequently provide better knowledge on how to account for the composition of guests in the hydrate structure. Hydrate phase equilibria plays a central role in all current areas of technological interest on hydrates. In addition, the PIs and graduate students will be involved in the outreach activities coordinated by Prof. Barbara Moskal at CSM. This program supports students and teachers from elementary to high school by providing direct contact with the PIs in an out of the classroom to prepare and deliver lessons and instruction materials. The current CSM GK-12 program serves three public schools systems in the Denver area with large representations of minority students. In collaboration with Prof. Moskal, the PIs will develop materials focused on the science of clathrate hydrates, their potential importance in energy technologies, as well as an introduction to molecular simulation. The PIs are also closely involved with the Center for Hydrate Research and the NSF Renewable Energy MRSEC at CSM. Clathrate hydrates is central in the research and educational activities of these centers, and the impact of this work will potentially extend to other timely projects in the centers by leveraging mutual interaction and collaboration among PIs and students