Duplex and Desalination — Is this the Future for Freshwater?

Water, water everywhere and not a drop to drink. This seeming paradox was written by Samuel Taylor Coleridge in The Rime of the Ancient Mariner and describes the unimaginable experience of slowly succumbing to thirst while stranded at sea and surrounded on all sides by life-giving H2O. Of course, the high concentration of salt and certain minerals found in seawater makes it undrinkable - any seawater that isn’t immediately rejected by the body will actually exacerbate the symptoms of extreme thirst. The only way to make it drinkable is to get the salts out - a process known as desalination.

One method of desalination is Multi-Effect Distillation (MED), which can be carried out in two temperature gradients (where the lower gradient is used, the process is referred to as LT-MED, with LT standing for Low Temperature). MED requires an array of dedicated, bespoke equipment to achieve success and this equipment must be particularly robust in order to withstand the extreme corrosive conditions experienced when working with seawater. This is where stainless steel takes centre stage.

Desalination via MED - The Process

• Seawater enters the MED evaporator, which comprises a number of consecutive cells (also known as ‘effects’). Each cell contains a bundle of tubes aligned horizontally and is maintained at a pressure & temperature slightly lower than the preceding cell, ranging from hot/high pressure in the first to cold/low pressure in the last

• The top of the first tube bundle is sprayed with seawater which moves from tube to tube utilising the effect of gravity

• Steam is introduced to the inside of the tubes to provide heat

• The tubes, usually made from Grade 316 L stainless steel, are externally cooled by make-up flow, causing fresh water to distil within the tubes

• Latent heat from condensation partly evaporates seawater outside the tubes, causing the remaining liquid to concentrate into a thick brine at the bottom of the cell

• Vapour raised by evaporation is lower in temperature than heating steam but is still warm enough to be used as a heating medium for the following effect, starting the process again.

• In the final cell, produced steam is caused to condense in a heat-exchanger, which is cooled by seawater and features an outlet where some of the remaining warmed liquid can be returned to make-up - the rest is rejected back into the sea

• The brine and distillate (freshwater) are collected from each cell into the final cell, from where they are extracted using centrifugal force

Though Grade 316 L stainless steel is currently the most common, highly-alloyed 254 SMO has been more recently used at several such plants in Saudi Arabia. Using duplex grades for such components as screws for MED applications is now a possibility and will enable exceptional cost savings in the coming years. In addition newly developed 100% waterproof rivets combining 316 and Duplex solve the problem of both leakage and corrosion - as our need for fresh water becomes ever greater, the technology could not have become available at a more appropriate time.

For the latest information on Duplex stainless steel screws and waterproof rivets check out the product page and call or chat live with our team to discuss your unique requirements.