Water utilities and developers

Desalination

Multi stage flash

Multi Stage Flash (MSF) The MSF process was invented and patented by Weirs of Cathcart, Scotland in 1957 and saw significant development and wide application throughout the sixties.

Heated brine is introduced to a chamber or ‘stage’where the pressure is below the saturated vapour pressure of the brine at that temperature. Immediate conversion to the vapour phase (flashing) takes place and the resulting vapour passes to the next stage.

After passing through a demister, a proportion of this vapour is then condensed on the tubes of the second stage, giving up its latent heat of vaporisation to the feed stream. The feed stream is heated progressively in each stage before passing through the brine heater used to feed the first stage.

The condensate collected in each stage forms the product, and the whole process is driven by the sub-atmospheric pressure gradient through the stages, generated at start-up by a steam ejector, but maintained by progressive condensation. The MSF process benefits from both the economy of scale and of its ability to operate on low grade steam, resulting in its widespread use worldwide.

Of the current world desalination capacity of approximately 30,000MLD, more than 80% is produced by the MSF process.

Other thermal processes include: Vapour Compression,where heat is supplied through mechanical compression of the vapour; Solar Distillation,where heat is provided by solar radiation; membrane distillation,where water vapour is diffused through a membrane impermeable to water and condensed on the liquid side; and fractional crystallisation or freezing,which is thermodynamically less energetic than processes where the water is heated.

Although these processes find novel application in special circumstances, none can yet be scaled efficiently to full commercial operation. Most large scale thermal desalination plants have relied upon the availability of low cost sources of heat and have therefore flourished where this is available – generally at electric power generation facilities. Recent attention given to the potentially detrimental effects of global warming resulting from the burning of fossil fuels has rekindled an interest in other sources of heat.

The most significant (and ironically, perhaps the most ‘environmentally friendly’) of these is nuclear energy,where there is considerable scope for much wider use in desalination. From 1973 until 1998, the BN-350 Fast Breeder Reactor at Aktau in Kazakhstan successfully produced 135MW of electricity and 80MLD of potable water from the Caspian Sea by MED.

The choice of reactor was deliberate, since the weapons grade plutonium was a desired by-product. It was this choice which has resulted in the lengthy (and environmentally not-so-friendly) decommissioning process currently being undertaken. India has provided nuclear power to a SWRO plant, and Japan has also successfully used pressurised water reactors for the generation of electricity for use in thermal desalination plants.