Multi- Desalination Process

Subject: Re: Multi- Desalination Process Thu Feb 11, 2010 2:30 pm

The once-through flash type evaporator uses the sea-water flow both for purposes of cooling (sea-water is introduced into the evaporator at the sea temperature and is rejected at the brine temperature) and production of distillate (by flashing from the outlet temperature of the brine heater to the brine extraction temperature). This has two consequences on plant design:

* The whole sea water flow being heated to high temperature, it has to be treated with anti-scale which increases operating costs.
* As the sea water flow cannot be decreased below values allowing safe working conditions, the stages must be designed for winter operation, leading to an increased evaporator volume and thus increased investment costs.

These two points have led to the separation of the two functions (cooling and production).

The cooling sea-water flows through the condensers of the two (or generally three) last stages, named "heat reject section". Upon leaving the evaporator, part of the warmed water is rejected to the sea, part is used as the make-up for the plant. Only this part of the water is treated instead of the whole cooling water. The production is insured by the brine recycling flow that is drawn from the last stage towards the condensers of the other stages, named "heat gain section", and then to the brine heater.

The warmed water leaving the heat reject section may be used in winter to warm up the cooling sea-water, thus enabling the evaporator volume to be designed for a reasonably high temperature.

MSF plants with brine recycling are widely used all over the world. Once-through desalination plant should only be used for small plants (when the cost of the chemicals is not of great importance) and in areas where the temperature of the sea-water remains approximately constant throughout the year.

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