Multi-Effect Distillation (MED) plants for seawater desalination: thermodynamic and economic improvement

The growing global demand of fresh water coupled with the increasing interest in renewable energy and in waste heat recovery has resulted in a growing attention to the Multi-Effect Distillation (MED) desalination process because it requires relatively low temperature sources and maximum Top Brine Temperature (TBT) of 70-90°C. The study of MED configurations that could increase the efficiency and reduce desalted water cost of production is an actual field of research. Differently from previous studies present in literature, a Parallel/Cross flow (P/C) MED was studied with variable feed mass flows in each effect to obtain the maximum allowable Recovery Ratio (RR). The maximum allowable RR is related to the need of avoiding scaling problems due to gypsum precipitation (function of the effect temperature and of the feed water salinity). Starting from a base P/C MED, several configurations have been simulated to find the best ones from a thermodynamic and economic point of view. The simulations have been conducted through an Aspen Plus model by varying the Bottom Brine Temperature (BBT) and the TBT. Configurations with a preheater for each effect showed the highest increase of Performance Ratio (PR) relative to the base configuration, in particular for the highest TBT and the lowest BBT (+ 10 % with TBT=75°C and BBT=35°C for a 4 effects-MED) due to the better exploitation of the energy content of desalted water streams