Search LMGS and measuring phase equilibrium for structure-H hydrates


K. Tezuka1, I. Kobayashi1, T. Taguchi1, S. Alavi2, A. Sum3, S. Takeya4 and R. Ohmura1

1Department of Mechanical Engineering, Keio University, Japan
2Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada
3Center for Hydrate Research, Chemical Engineering Department, Colorado School of Mines, Golden, Colorado, U.S.A.
4National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan

Keywords: clathrate hydrate
property: phase equilibrium
material: methane; bromocyclohexane; 2-bromobutane

Structure-H hydrate forms with two different guest substances: one is a small molecule guest substance, such as methane, Xe, and the other is a relatively large molecule guest substance, such as methylbutanes. The latter substance is denoted as a large molucule guest substance(LMGS).

A large number of phase equilibrium data have been reported for various LMGSs’ with methane. At present, 1,1-dimethylcyclohexane is known to be the LMGS that formed at the lowest equilibrium pressure in the presence of methane as the small molecule guest  (Thomas and Behar, 1995).  Specifically, the equilibrium pressure of the hydrate is 2.0 MPa at 280.2 K, which is substantially lower than that of structure-I simple methane hydrate at the same temperature, 5 MPa. However, there is no comprehensive understanding on the correlation between the thermodynamic stability of structure H hydrates and the chemical species of LMGSs.

In this study, we calculated the molecular sizes and the molecular volumes of LMGSs reported in the literature using Gaussian03.  We then investigated the correlation among the molecular sizes, the molar mass of LMGSs and the equilibrium pressures. The results suggest that the lowest equilibrium pressure for structure-H hydrates is formed with LMGS having C-C length of 0.45 nm (or molecular volume of 190 Å.  Also, the lower equilibrium pressure may be available with LMGS having larger molar mass. Based on these results, we tested the replacement between methylgroup and bromine atom in LMGS, which results in the increase in the molar mass but keep the molecular size.  Specifically, we measured the phase equilibrium for the hydrates formed with methane and bromocyclohexane or 2-bromobutane, which may be viewed as methylcyclohexane and 2-methylbutane with the replacement of methylgroup with bromine. Their equilibrium pressures are higher than with methane and methylcyclohexane or 2-methylbutane at same temperature.  Discussion is given on the possible molecular mechanisms of this increase in the equilibrium pressure due to the methylgroup-Br replacement.

References
  1. Thomas, M.; Behar, E. Proc. 73rd GPA Convention, 1995, New Orleans, LA, March 7-9

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