Development of a Polymer-Composite-Based Evaporator for Brine Concentration

The recovery of valuable metals and minerals from concentrated seawater is gaining increasing importance. To concentrate the brine from seawater desalination plants in robust evaporators, corrosion-resistant and costly metals must be used. The industry is therefore seeking corrosion- resistant and cost-effective alternatives. The main objective of this work was the development of a fully polymer-composite-based evaporator for brine concentration using the multiple-effect distillation (MED) process. Extruded polymer composite tubes, manufactured via an advanced process enabling high filler volume fractions and enhanced particle orientation, were investigated as candidate evaporator tubes. Polypropylene (PP) and polyphenylene sulphide (PPS) matrices were filled with graphite (GR) flakes to a volume fraction of 50 %, achieving through-wall thermal conductivities of 6.5 W/(m K) and 4.5 W/(m K) at 25 °C, respectively, values comparable to titanium alloys and representing record conductivities for extruded polymer composite tubes. Finite element simulations and experimental studies identified filler orientation, volume fraction, and particle connectivity as key parameters governing composite thermal conductivity. Mechanical testing confirmed that both PP-GR and PPS-GR tubes provide sufficient rigidity for MED operating conditions, with PPS-GR offering higher temperature capability. Surface analysis showed that untreated tubes were hydrophobic, resulting in incomplete wetting in falling-film operation. Flame and plasma treatments significantly improved wettability. Pilot-scale falling film evaporator tests with flame-treated composite tubes achieved overall heat transfer coefficients of approximately 1,500 W/(m² K), compared with 2,770 W/(m² K) for stainless steel. Scaling experiments with concentrated seawater and calcium sulphate solutions demonstrated a lower fouling propensity for polymer composites than metals, attributed to weaker deposit adhesion. The lightweight polymer composite tubes with enhanced thermal conductivity allow the use of polymer-based materials for tube plates and other structural components, leading to significant weight cost reductions. This work includes the development of a novel, fully polymer-based evaporator that integrates polymer composite tubes with structural components, such as shell and tube plates, made of fibre-reinforced vinylester. The results of this work demonstrate the viability of polymer-composite-based evaporators for a wide range of heat transfer applications in corrosive environments.