Mechanism elucidation and scaling control in membrane distillation using 3D printed carbon nanotube spacer

Membrane scaling is a barrier to membrane distillation (MD). In this study, 3D-printed carbon nanotube (CNT) spacer was used to investigate its capability for mitigating membrane scaling during MD and to elucidate the scaling mechanism experimentally and theoretically. CNT spacer was tested under temperature-dependent calcium sulfate scaling conditions, and optical coherence tomography (OCT) and scanning electron microscopy (SEM) were used to measure scaling quantitatively. CNT spacer exhibited unique membrane scaling mechanism, where only a 37% reduction (29 Lm2h-1) in the initial flux was achieved, even above a volume concentration factor (VCF) of 4. On the other hand, the membrane with a polylactic acid (PLA) spacer (controls) entirely lost flux before reaching a VCF of 3.5. Interestingly, bubble formation was observed in CNT spacer, which could be attributed to the enhanced flux and vaporization rate on membrane surface in the presence of rough-surfaced CNT spacer. Bubbly flow along the membrane channel with CNT spacer can potentially reduce surface scaling on membrane during MD. Moreover, due to the surface roughness of CNT spacer, the initial nuclei might be detached more easily from CNT spacer surface than from smooth PLA surface and grow further into larger crystals in the bulk, resulting in reduced dissolved solutes in the solution. This phenomenon was indirectly corroborated by comparing the experimentally measured fluxes and theoretically computed values from our mechanistic model of MD-crystallization developed in this study. Therefore, this study revealed that CNT spacer with rough surfaces can potentially have benefit of mitigating membrane scaling during MD.