Fluid Science & Resources, Department of Chemical Engineering, School of Engineering, University of Western Australia, 35 Stirling Hwy, Crawley 6009, Australia.
Future Energy Exports Cooperative Research Centre, 35 Stirling Highway, Perth, Western Australia 6009, Australia.
Langmuir. 2023 Apr 4;39(13):4631-4641. doi: 10.1021/acs.langmuir.2c03376. Epub 2023 Mar 23.
Natural gas containing trace amounts of water is frequently liquefied at conditions where aqueous solids are thermodynamically stable. However, no data are available to describe the kinetics of aqueous solid formation at these conditions. Here, we present experimental measurements of both solid formation kinetics and solid-fluid equilibrium for trace concentrations of (12 ± 0.7) ppm water in methane using a stirred, high-pressure apparatus and visual microscopy. Along isochoric pathways with cooling rates around 1 K·min, micron-scale aqueous solids were observed to form at subcoolings of (0.3-8.6) K, relative to an average equilibrium melting temperature of (253 ± 1.9) K at (8.9 ± 0.08) MPa; these data are consistent with predicted methane hydrate dissociation conditions within the uncertainty of both the experiment and model. The 36 measured formation events were used to construct a cumulative formation probability distribution, which was then fitted with a model from Classical Nucleation Theory, enabling the extraction of kinetic and thermodynamic nucleation parameters. While the resulting nucleation parameter values were comparable to those published for methane hydrate formation in bulk-water systems, the observed growth kinetics were distinctly different with only a small percentage of the water in the system converting into micron-scale solids over the experimental time scale. These results may help explain how cryogenic heat exchangers in liquefied natural gas facilities can operate for long periods without blockages forming despite being at very high subcoolings for aqueous solids.
天然气中常含有微量水，这些水在热力学上稳定的条件下被液化。然而，目前还没有数据可以描述这些条件下的水固形成动力学。在这里，我们使用搅拌高压设备和可视化显微镜，对甲烷中（12±0.7）ppm 痕量水的固形成动力学和固-液平衡进行了实验测量。在等容路径和大约 1 K·min 的冷却速率下，观察到微米级的水固在过冷度为（0.3-8.6）K 时形成，相对于（8.9±0.08）MPa 下平均平衡熔点（253±1.9）K；这些数据与甲烷水合物分解条件的预测一致，实验和模型的不确定性都在其中。36 次测量的形成事件被用来构建累积形成概率分布，然后用经典成核理论模型进行拟合，从而提取出动力学和热力学成核参数。虽然得到的成核参数值与在全水体系中甲烷水合物形成的成核参数值相当，但观察到的生长动力学明显不同，只有系统中一小部分水在实验时间尺度内转化为微米级固体。这些结果可能有助于解释为什么在非常高的水固过冷度下，液化天然气设施中的低温热交换器可以长时间运行而不会形成堵塞。
