vdWP-FL: An Improved Thermodynamic Theory for Gas Hydrates with Free-Energy Contributions due to Hydrate Lattice Flexibility

A modification
of the original van der Waals and Platteuw (vdWP) theory that accounts
for free-energy contributions due to the vibrational and librational
movement of water molecules in the hydrate lattice is presented. The
modified theory is labeled vdWP-FL. In our previous work, we had presented
a method to compute these contributions to the partition function.
This was successfully applied to simulated phase equilibria of various
gas hydrates. In this work, we apply the vdWP-FL theory to experimental
data of gas hydrate phase equilibria and recompute the guest–water
potentials. The empty hydrate reference properties are directly computed
from molecular simulations. In this implementation of the vdWP-FL
theory, only one parameter per guest molecule per cavity type is regressed
from the experimental data on gas hydrates. The gas hydrates chosen
for this study are methane, ethane, carbon dioxide, propane, iso-butane,
and the hydrates formed by their binary mixtures. The vdWP-FL theory
gives accurate predictions of the dissociation temperatures and pressures
of the above gas hydrates and their mixtures. In addition, it also
predicts the hydrate cage occupancy accurately.

本文提出了一种针对原始范德瓦尔斯-普拉特尤（van der Waals and Platteuw, vdWP）理论的修正方案，该修正理论考虑了水合物晶格中水分子的振动与摇摆运动所带来的自由能贡献。该修正理论被命名为vdWP-FL理论。在此前的研究中，我们已提出一种用于计算配分函数中上述自由能贡献的方法，并将其成功应用于多种气体水合物的模拟相平衡计算。本文将vdWP-FL理论应用于气体水合物相平衡的实验数据，并重新计算了客体-水分子间的相互作用势。空水合物的参考性质可直接通过分子模拟获得。在该vdWP-FL理论的实现框架中，仅需针对每种空腔类型的每种客体分子，从气体水合物实验数据中回归得到一个参数。本研究选取的气体水合物体系包括甲烷、乙烷、二氧化碳、丙烷、异丁烷及其二元混合体系形成的水合物。vdWP-FL理论可准确预测上述气体水合物及其混合体系的解离温度与压力，同时还能精准预测水合物的笼腔占有率。