Abstract

Membrane distillation (MD) is a desalination technique that uses a membrane to thermally separate potable water from sea or brackish water. The mass transport processes through the membrane are commonly described by the dusty gas model. These processes are modeled assuming uniform, ideally cylindrical capillaries and are adjusted for the membrane geometry by including porosity and tortuosity. The tortuosity is usually set to 2 or is used as an adjusting parameter to fit theoretical models to experimentally measured data. In this work, ptychographic X-ray computed tomography is employed to map the three-dimensional (3D) structure of three commercial state-of-the-art PTFE membranes in MD. The porosity, tortuosity and permeability (viscous flow coefficient) of the samples are computed using the lattice Boltzmann method. The intrinsic permeability is compared to the dusty gas model and an apparent permeability is proposed which is corrected for Knudsen slip effects at the membrane structure.

Article Highlights

• 3D structure of membranes for distillation measured at full height at an unprecedented detail using X-ray ptychography for the first time.

• Comparison of the dusty gas model to 3D direct numerical simulation: permeability and Knudsen effects.

• Membrane characterization and calculation of the hydraulic tortuosity factor from 3D flow field simulations.

中文翻译：

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X射线声像图的三维膜成像：膜蒸馏的膜传输性能的确定。

摘要

膜蒸馏（MD）是一种脱盐技术，该技术使用膜将饮用水与海水或微咸水热分离。通常通过粉尘气体模型描述通过膜的传质过程。对这些过程进行建模时，假设使用均匀的理想圆柱形毛细管，并通过包括孔隙率和曲折度来调整膜的几何形状。弯曲度通常设置为2或用作调整参数，以使理论模型适合实验测量的数据。在这项工作中，使用了X射线计算机断层扫描技术绘制了MD中三种最先进的PTFE膜的三维（3D）结构图。样品的孔隙率，曲折度和渗透率（粘流系数）是使用格子Boltzmann方法计算的。

文章重点

• 首次使用X射线谱图技术以前所未有的细节在全高度测量了用于蒸馏的膜的3D结构。

• 含尘气体模型与3D直接数值模拟的比较：渗透率和克努森效应。

• 从3D流场模拟中膜表征和水力曲折系数的计算。