Hester K C, Huo Z, Ballard A L, Koh C A, Miller K T, Sloan E D
Center for Hydrate Research, Colorado School of Mines, Golden, Colorado 80401, USA.
J Phys Chem B. 2007 Aug 2;111(30):8830-5. doi: 10.1021/jp0715880. Epub 2007 Jul 11.
Knowledge of thermal expansivity can aid in the understanding of both microscopic and macroscopic behavior of clathrate hydrates. Diffraction studies have shown that hydrate volume changes significantly (as much as 1.5% over 50 K) as a function of temperature. It has been demonstrated previously via statistical mechanics that a minor change in hydrate volume (e.g., a 1.5% change in volume or 0.5% change in lattice parameter) can lead to a major change in the predicted hydrate formation pressure (e.g., >15% at >100 MPa for methane). Because of this sensitivity, hydrate thermal expansivity measurements, for both Structures I and II with various guests, are needed help quantify volume distortions in hydrate lattices to ensure accurate hydrate phase equilibria predictions. In addition to macroscopic phase equilibria, the thermal expansion of different hydrates can give information about the interactions between the guest molecules and the host lattice. In this work, the hydrate lattice parameters for four Structure I (C2H6, CO2, 47% C2H6 + 53% CO2, and 85% CH4 + 15% CO2) and seven Structure II (C3H8, 60% CH4 + 40% C3H8, 30% C2H6 + 70% C3H8, 18% CO2 + 82% C3H8, 87.6% CH4 + 12.4% i-C4H10, 95% CH4 + 5% C5H10O, and a natural gas mixture) systems were measured as a function of temperature. The lattice parameter measurements were combined with existing literature values. Both sI and sII hydrates, with a few exceptions, had a common thermal expansivity, independent of hydrate guest. Many guest-dependent correlations for linear thermal expansivity have been proposed. However, we present two guest-independent, structure-dependent correlations for sI and sII lattices, which have been developed to express the normalized hydrate lattice parameters (and therefore volume) as a function of temperature.
了解热膨胀系数有助于理解笼形水合物的微观和宏观行为。衍射研究表明，水合物的体积随温度变化显著（在50 K范围内变化高达1.5%）。此前通过统计力学已经证明，水合物体积的微小变化（例如，体积变化1.5%或晶格参数变化0.5%）会导致预测的水合物形成压力发生重大变化（例如，对于甲烷，在>100 MPa时变化>15%）。由于这种敏感性，需要测量结构I和结构II与各种客体的水合物热膨胀系数，以帮助量化水合物晶格中的体积畸变，确保准确预测水合物相平衡。除了宏观相平衡外，不同水合物的热膨胀还可以提供有关客体分子与主体晶格之间相互作用的信息。在这项工作中，测量了四种结构I（C2H6、CO2、47% C2H6 + 53% CO2和85% CH4 + 15% CO2）和七种结构II（C3H8、60% CH4 + 40% C3H8、30% C2H6 + 70% C3H8、18% CO2 + 82% C3H8、87.6% CH4 + 12.4% i-C4H10、95% CH4 + 5% C5H10O和一种天然气混合物）体系的水合物晶格参数随温度的变化。将晶格参数测量值与现有文献值相结合。除了少数例外，结构I和结构II水合物都具有共同的热膨胀系数，与水合物客体无关。已经提出了许多与客体相关的线性热膨胀系数相关性。然而，我们提出了两种与客体无关、与结构相关的结构I和结构II晶格相关性，这些相关性已被开发用于将归一化的水合物晶格参数（从而体积）表示为温度的函数。
