Coupled mechanical and hydrodynamic 3D modelling to evaluate membrane intrusion impact on pressure drop in reverse osmosis permeate channels

In reverse osmosis spiral wound membrane modules, the applied pressure causes membrane intrusion in permeate channels, altering the permeate flow. The present study developed a 3-D mechanical and fluid dynamics simulation framework, applied at the small scale of a periodic unit of membrane-permeate spacer assembly. The feed spacer was not simulated and only one membrane placed above the permeate spacer was analysed (one-sided intruding system, like in the used experimental flow cell). The mechanical model computed the deformation of the assembly under different pressures, taking an undeformed spacer geometry accurately determined by CT scanning. Detailed deformed permeate channel configurations were obtained. The mechanical characteristics of the membrane and permeate spacer were estimated by making use of membrane intrusion experimental data obtained by microscopic quantitative imaging with optical coherence tomography. The computed deformed membrane-spacer geometries were used in fluid dynamics calculations. An excellent agreement was found between numerical and experimental data on pressure drop versus velocity in deformed channels. The membrane intrusion under pressure caused a large reduction in permeate channel porosity and thus a strong increase of pressure loss. This study reveals the importance of considering mechanical deformations in computing performance indicators while designing pressure-based membrane separation modules.