Potential of membrane distillation in seawater desalination: Thermal efficiency, sensitivity study and cost estimation

In this work, an extensive analysis on direct contact membrane distillation (DCMD) performance was developed to estimate the mass flux and the heat efficiency, considering transport phenomena, membrane structural properties and most sensitive process parameters, with the aim to provide optimization guidelines for materials and methods. The results showed that an increase of the temperature gradient resulted in the enhancement of both transmembrane flux and thermal efficiency. The investigation of the effects of membrane properties confirmed that better DCMD performance was achieved when using polymeric membranes characterized by low thermal conductivity (flux and thermal efficiency declined by 26% and 50%, respectively, when increasing thermal conductivity from 0.1 to 0.5 W/m K), and high porosity. An optimal thickness value (around 0.7 mm) was identified when operating at low temperature gradient (<5 °C). However, at higher temperature gradient (>10 °C), increasing the membrane thickness from 0.25 to 1.55 mm resulted in a flux decay of about 70% without a significant improvement in thermal efficiency. Exergy analysis, sensitivity study and economical evaluation were carried out to assess the feasibility of DCMD process. For DCMD with heat recovery, the estimated water cost was $1.17 m^-3, which was comparable to the cost of water produced by conventional thermal processes: i.e. around $1.00 m^-3 for multiple effect distillation (MED) and $1.40 m^-3 for multi-stage flash (MSF). However, significant savings are expected when using a low-grade thermal energy source, decreasing the cost of DCMD to values approaching the cost of water produced by reverse osmosis (RO), which is about $0.50 m^-3.