Composite Conducts Heat to Remove Salt From Seawater

Desalination attempts for converting seawater to fresh, drinkable water are on the rise. The decision to pursue or not pursue desalination projects is a ballot item right now in Santa Cruz, Calif., the city nearest to the town I live in. It's a pressing issue in many parts of the world, including here where the aquifer has been shrinking for a couple of decades.

A new composite material invented by researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) may help bring down the cost of some desalination plants. These processing plants require highly specialized, expensive infrastructure, including pipelines made of titanium and various forms of high-alloy steel that resist the corrosive action of salt water. Although pipeline cost isn't the only expense, expense in general is one of the objections to desalination.

The most common desalination method is multi-stage flash distillation, a form of thermal distillation. In thermal distillation desalination methods, the pipes must also be heat-resistant, since this process requires them to be heated until water sprayed on them boils and evaporates, leaving behind salt and chemical residues. Pipes in desalination plants must therefore be highly durable, corrosion-resistant, heat-resistant, and long-lasting. They must also have a surface that can be easily coated with seawater, so the fresh water evaporates correctly.

The Fraunhofer IFAM's polymer composite conducts heat and can be produced in continuous lengths, which the researchers say will make it less expensive than the specialty metals it can replace. Although a plastic, the composite conducts heat because it contains copper microfibers, about 50 percent by volume. "This does not change the processing properties of the composite, and it can still be processed as any other polymer would," said Arne Haberkorn, an IFAM scientist, in a press release.

The material has been developed, and the researchers exhibited it at the Composites Europe 2012 trade show in Dusseldorf, Germany, October 9 to 11. Their next step is to optimize its thermal conductivity by testing it in a pilot seawater desalination plant.

During testing, they expect to measure the composite's thermal conductivity, observe how much microorganism-based coating forms on the pipes, and see to what extent the pipes corrode in a salty environment. In the pilot plant's evaporation process, hot gas heated to 70C will run through the pipelines. This temperature is lower than some of the highest temperatures in multi-stage flash distillation, which can run up to 120C. The benefit of lower temperatures is that the material doesn't corrode as quickly, there's less deposit buildup on the pipes, and the inside and outside pressure differential is much less. This results in pipes that last longer and a process that requires less maintenance.

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