| Budget Amount *help |
¥1,700,000 (Direct Cost: ¥1,700,000)
Fiscal Year 2003: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 2002: ¥900,000 (Direct Cost: ¥900,000)
|
|---|
| Research Abstract |
The molecular dynamics (MD) method was applied to CH_4-CO_2 mixed hydrate (structure I, Pm3n) in order to produce and predict the compositional dependence of crystallographic and thermodynamic properties.
The following three types of mixed hydrate crystals were prepared for the simulation:
1.The CH_4 is completely partitioned into S-cage, and CO_2 into M-cage.
2.The CH_4 and CO_2 are evenly distributed to S- and M-cages (Disordered structure).
3.The CO_2 is completely partitioned into S-cage, and CH_4 into M-cage.
The system contains 216 gas molecules (=54 in S-cage + 162 in M-cage) and 1242 H_2O molecules. The interatomic potential model employed in this study is an empirical one, which can reproduce the dielectric constant, diffusion coefficient, and density for H_2O, the vibration spectrum, density, and structure for CH_4 and CO_2. The MD calculation was carried out under some pressures and temperature with the step time of 0.4 fs.
The characteristic results are as follows:
1.The compositional dependence of lattice parameter
The lattice parameter varies systematically with the CO_2 concentration [=CO_2/(CH_4+CO_2)] in M- and S-cages. The concentration of CO_2 in S-cage largely affects the lattice parameter.
2.The compositional dependence of compressibility
The crystals with the enrichment of CO_2 in S-cage have high compressibility. The bulk modulus of CO_2 hydrate is about 10% smaller than that of CH_4 hydrate.
3.The compositional dependence of molar enthalpy
The structure with the enrichment of CO_2 into M-cage is more stable than the disordered structure, where CH_4 and CO_2 are evenly distributed to both cages. This is consistent with the result of cage occupancy measurements by use of Raman spectroscopy (Nakano and Ohgaki, 2000).
|
