Integrated internal feed preheating design for modular multieffect membrane distillation

Abstract

A modular permeate gap forward-feed multi-effect membrane distillation (PG-MEMD) system with integrated internal feed preheating channels was built and experimentally compared against the baseline MEMD design. Inlet feed first flows through the preheat channels in the condensation plates from the final low-temperature effect backward before being introduced into the first effect for evaporation. Hence, external high-grade heat input to the first effect is used toward phase change rather than toward sensible heating, improving overall pure water production and energy efficiency by, on an average, approximately 80%. A maximum recovery ratio (RR) of about 21% was achieved in a single pass through the benchtop distillation device, along with a gained output ratio (GOR) of ∼1.3. The system was tested at various feed salinities, ranging from brackish to hypersaline levels, showing a performance drop of around 35% under hypersaline conditions. The internal preheating design consistently outperforms the baseline non-preheating version under all conditions. Adding fins to the feed and preheat channels led to an improvement of ∼18% and ∼14% in RR and GOR, respectively. The addition of fins also reduced the first-effect temperature, and can therefore enable the addition of a greater number of effects for further improvement in the energy efficiency.