Polymeric hollow-fiber heat exchangers for thermal desalination processes

Metallic shell-and-tube heat exchangers used in thermal desalination require huge capital investments, suffer from corrosion/erosion, create heavy metal pollution, and display a large footprint. This paper has explored their potential replacement by polymeric solid hollow fiber-based heat exchangers. Using solid hollow-fibers of polypropylene (PP) (wall thickness 75 μm, outside diameter 575 μm) a number of heat exchangers were fabricated in the laboratory (heat exchange area, 195-960 cm²) and at a commercial manufacturing facility (heat exchange area, 0.15-0.44 m²). The heat transfer performances of these devices were studied for a hot brine (4% NaCl, ca. 80-98 °C)-cold water (8-25 °C) system as well as for a steam (101-113 °C)-cold water (8-25 °C) system; these systems are typically encountered in thermal desalination plants. Overall heat transfer coefficient values of as much as 2000 W/(m² K) were achieved. This is close to the limiting value imposed by the PP wall thickness, namely, 2660 W/(m ² K). Heat exchangers built out of solid poly(ether ether ketone) (PEEK) fibers performed almost as well. Higher heat transfer coefficients were obtained by using porous asymmetric polyethersulfone hollow fibers whose internal diameter was coated by two consecutive layers of polyamide and silicone to make them impervious to moisture. A mathematical model has been developed to describe the solid hollow fiber heat exchanger performance and was proven a good predictor of heat transfer performance in such devices. Compared to metallic exchangers, these heat exchangers weigh much less, have an order of magnitude larger surface area per unit volume, and are likely to be considerably cheaper. Small polymeric heat exchange devices having an effective length less than 30.5 cm (∼1 ft.) achieve efficiencies close to 1, provide NTU values close to 4, and have HTU values as low as 5 cm. Further their conductance/volume values are as much as 2-15 times larger than metal heat exchangers. In addition, these devices have low flow pressure drops as low as 1 kPa/NTU compared to 30 kPa/NTU in conventional metal heat exchangers.