POLYMER FILM HEAT EXCHANGER FOR SEAWATER DESALINATION : PREVENTION AND CLEANING OF FOULING DEPOSITS

Polymer film heat exchangers show a considerable potential as a cost-efficient apparatus for thermal seawater desalination plants. However, the occurrence of mineral scale formation on polymeric heat transfer surfaces requires basic investigations in order to improve existing design criteria. A polymer film heat exchanger (PFHX) concept, as well as the scaling affinity of the polymers compared to the benchmark stainless steel, is discussed. The scaling kinetics on polymer surfaces are slower compared to stainless steel, resulting in an extension of a save operation period before cleaning of about a factor of 10. Once the threshold of heat transfer due to fouling is reached, a fast cleaning of PFHX surface is accomplished by a hydraulic shock without any use of chemicals. INTRODUCTION In view of steadily increasing prices for high performance metal alloys, polymers are becoming a serious alternative construction material in apparatus design for niche applications. In the past polymeric heat exchangers had been developed to condensate corrosive media (Bigg et al., 1989; Reay, 1988), designed as bundled tubes and later as compact or foil heat exchangers (Zaheed and Jachuck, 2004; Cevallos et al. 2012). A novel PFHX concept became also feasible due to new material developments of PEEK (polyetheretherketone), which offers improved mechanical properties (Christmann et al., 2012). The application of thin polymer films as heat transfer surface is a promising concept for industrial use. Christmann et al. (2013) described the apparatus concept of such a polymer film heat exchanger with its thermal and mechanical limits. As to that, further investigations on parameters influencing mineral scale deposition on polymeric heat transfer surfaces are required. The scope of the presented work is directed to scaling effects under multi-effect-distillation (MED) conditions occurring in thermal desalination plants. In that respect the fouling affinity of different minerals is discussed, as well as strategies for mitigation and cleaning of deposits. In detail it concentrates on the comparison of stainless steel (1.4571) as a benchmark in apparatus design and the commercially available thermoplastic polymers polyetheretherketone (PEEK) and its modifications (Victrex Europa GmbH) as well as polysulfone (PSU, Dr. D. Müller GmbH). Studied model salts are supersaturated calcium sulfate and calcium carbonate solutions. POLYMER FILM HEAT EXCHANGERS Material requirements in heat exchanger design depend on the application but address thermal, mechanical and chemical properties. Polymeric materials feature a high corrosive stability, but usually have very low thermal conductivities. The application of Nusselt correlations for falling film heat transfer on vertical planes illustrates the impact of thermal resistance of heat conduction on the overall heat transfer coefficient U. The non-dimensional Reynolds, Prandtl and Nusselt numbers are necessary to describe the heat transfer of condensation and evaporation side and are defined as follows (VDI, 2006; Schnabel and Schlünder, 1980):