The potential of hollow fiber vacuum multi-effect membrane distillation for brine treatment

[Display omitted] •A novel multi-effect hollow fiber vacuum membrane distillation system was developed.•A condensation-convection-distillation heat and mass transfer mechanism was proposed.•Internal heat recovery was realized with a hollow 3-D printed aluminium baffle.•The system reduced the energy consumption by ~60% compared to a single effect design.•Enlarged system (~1 m2 membrane per effect) could achieve a flux of ~4.5 LMH. Vacuum membrane distillation can extract pure water from degraded sources, but it requires relatively high energy inputs as compared to other thermal-driven technologies. As a commercially successful example, Memsys Water Technologies GmbH has addressed this key limitation by developing a flat sheet-based vacuum membrane distillation module where the latent heat recycled internally by using multiple distillation effects. In this paper, we propose an alternative hollow fiber-based design which also recycles the latent heat with multiple effects, but with an even more compact membrane packing format. The proposed design uses 3-dimensional printing to ‘unlock’ this configuration via a hollow aluminium alloy baffle which relies on its low thermal resistance to recover the latent heat effectively. The printed metal baffle (0.8 mm wall thickness) was calculated to have a very high conductive heat transfer coefficient, ~180 kW/m2K (surpassing even the ~20 μm polypropylene foil used in the Memsys module, which has a conductance of ~10 kW/m2K). Our experimental and theoretical results indicate that this design uses a condensation-convection-distillation heat and mass transfer mechanism which enables a three-effect system to reduce the energy consumption by ~60% over a single-effect design (i.e. from 672 kWh/m3 to 263 kWh/m3) for synthetic geothermal brine (~200 g/L salt concentration). Furthermore, the prototype reached a high average permeate flux of ~5.1 LMH and a salt rejection rate of >99.99%, approaching zero liquid discharge. Overall, this work suggests that hollow fiber membranes can indeed be used in a multi-effect mode and represents a promising new pathway for membrane distillation.