Polymeric Hollow-Fiber Heat Exchangers for Thermal Desalination Processes

Abstract

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 cm 2 ) and at a commercial manufacturing facility (heat exchange area, 0.15-0.44 m 2 ). 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/(m 2 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.

Recent research developments in polymer heat exchangers – A review

“…Song et al 53 proposed experiments and numerical simulation of solid polymeric hollow fibres heat exchangers (PHFHE) for thermal desalination process. Three types of polymeric hollow fibres (as shown in Figure 21) including solid PP, solid PEEK and asymmetric polyethersulfone (PES) with nonporous coatings were used to produce the heat exchangers.…”

Novel membrane contactor for CO 2 removal from flue gas by temperature swing absorption

“…The shell had two additional openings; one was connected (not shown) to a liquid absorbent reservoir kept mildly pressurized to prevent gas bubbling in the absorbent from the gas passing through the bore of the porous hollow fibers. We could have used inexpensive solid polypropylene hollow fibers [21,22] since as we will see later the stripping temperature employed here did not exceed 95-97 o C.…”

“…Otherwise the equilibration time is long. The polymeric solid PEEK hollow fibers having a thin wall transfer heat with great efficiency [21,22] and can rapidly heat the absorbent in the inter-fiber space if we introduce hot water or low temperature steam through their lumen. These solid hollow fibers can also absorb the exothermic heat of absorption during CO 2 absorption if cold water is passed through their lumen for isothermal absorption.…”

A PCM-water heat exchanger with polymeric hollow fibres for latent heat thermal energy storage: a parametric study of discharging stage

“…It was shown that the overall conductance per unit of volume of the PHFHE (around 1.2 · 10 6 W/m 3 K) is larger than in the case of conventional metallic shell-and--tube exchangers and slightly less than in the case of the plate heat exchangers. Song et al (2010) presented a lab study aimed at the use PHFHEs in the thermal desalination process. The metallic heat exchangers suffer from corrosive behaviour of salt water while the plastics withstand salt water quite well.…”