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

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... erimentally 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.