(236g) Prediction of Hydrogen Hydrates Equilibria under an Organic Compound

Around 50 g of hydrogen gas molecules is held in one liter of solid water cavities (5.3wt%) in the form of gas hydrates. It is synthesized at 2000-3000 bar and 240-249 K and can be preserved at 140K under ambient pressure [1]. Because of this excellent storage capacity, H2 hydrates can be used as a storage medium to secure the supply of H2. However, this technique can be economically competitive and widely applied only if H2 hydrates are formed at lower pressures. Recently, two research institutes discovered a low-pressure H2 hydrate in combined THF (tetra-hydrofuran) hydrates around 50 bar and 280K [2], and 120 bar and 270K [3]. Gas hydrates are crystalline compounds, also known as clathrates. The H2 molecules are entrapped in a cage like structure of hydrogen bonded water molecules. Their general formula is (G)n(H2O)p where G: Gas, n : number of gas molecules, p: number of water molecules. The design of gas hydrate storage systems needs an accurate prediction of dissociation pressures with temperature. We have proposed a multi-scale modeling for pure H2 hydrates by combining ab-initio calculations and statistical thermodynamics [4]. In this presentation, we will introduce an excess Gibbs energy model for low-pressure H2 hydrates under a THF aqueous solution.

THF enters only the large cavities of Structure-II clathrates while H2 can enter both small and large cavities with the occupancies of 2 and 4 respectively. The 2H2 / 4H2 molecules in the cavities are treated as rigid body clusters and the corresponding intermolecular potential parameters between water and H2 clusters are used to calculate the Langmuir constants of H2 clusters in the cavities. Then the excess Gibbs energy model along with the Lee-Holder cell distortion model and the vander Waals - Platteeuw statistical thermodynamic model is used to predict the dissociation pressures of H2-THF double hydrates. The agreement between the predicted dissociation pressures and the experimental dissociation pressures is quite good. Then, we will extend our proposed model to the gas hydrates formed from the mixtures containing H2, THF, and other gases.

References

1. W. L. Mao, H. K. Mao, A.F. Goncharov, V. V. Struzhkin, Q. Guo, J. Hu, J. Shu, R. J. Hemley, M. Somayazulu, and Y. Zhao, “Hydrogen Clusters in Clathrate Hydrate”, Science, 297, 2247-2249 (2002).

2. L. J. Florusse, C. J. Peters, J. Schoonman, K. C. Hester, C. A. Koh, S. F. Dec, K. N. Marsh, and E. D. Sloan, “Stable Low-Pressure Hydrogen Clusters Stored in a Binary Clathrate Hydrate”, Science, 306, 469-471 (2004).

3. H. Lee, J-W Lee, D. Y. Kim, J. Park, Y-T Seo, H. I. Zeng, L. Moudrakovski, C. I. Ratcliffe, and J. A. Ripmeester, “Tuning Clathrate Hydrates for Hydrogen Storage”, Nature, 434, 743-746 (2005).

4. J. W. Lee, P. Yedlappali, and S. Y. Lee, “Prediction of Hydrogen Hydrate Equilibrium by Integrating Ab-initio Calculations with Statistical Thermodynamics”, Journal of Physical Chemistry B, 110 (5), 2332-2337 (2006).