MSF Evaporation

MSF or Multiple Stage Flash is an evaporation technology that was the dominant method for sea water desalination before R.O. became a mature and reliable technology. To this day, very large MSF units are being operated particularly in the Middle East where entire countries rely on sea water desalination as their predominant source of potable water.

MSF is a thermal process in which the feed stream together with a certain recirculation stream is incrementally preheated in a series of the heat exchangers that are mounted inside of each of the flash stages which operate at ever decreasing temperature levels. Preheating takes place in counterflow to the flash sequence, at ever increasing temperature levels, by condensing all of the flash vapor generated in each stage. As the saline fluid rises through each stage of higher temperature, it will enter a final heater at a temperature that is somewhat lower than the one in the first flash stage. The final heater is heated with an external energy source (mostly steam, some solar, geothermal or waste heat sources) to a temperature which is above the flash temperature of the first stage. This final temperature differential roughly corresponds to the temperature differential between each of the flash stages all of which are approximately the same.

The flash sequence now starts at the top of the vessel. On its way down through each flash stage the saline fluid then releases heat to the extent that the preheater in that stage will absorb. From the lowest stage which also is the stage of lowest temperature, the saline fluid (brine) is then discharged, minus the recirculation portion of it.

The condensate (distillate) is captured separately in each stage such that it can not remix with the saline fluid in the system. The distillate then also is flashed down through the lower temperature stages on a separate path for the purpose of heat recovery before being cooled against the feed stream and discharged from the system.

The merit of MSF is its energy efficiency accomplished with a relatively simple and reliable system. As in all evaporation systems, approximately 570 kcal are needed to vaporize (1) kg of water. This would be the case for a single stage system, without considering any losses. Adding stages will reduce energy consumption to the single stage consumption divided by the number of stages plus minor losses. An (8)-stage system therefore would consume 570/8 kcal + 12% = 80 kcal to distill the (1) kg of water from the saline fluid.

While the efficiency gains are dramatic for the first few stages added, higher numbers of stages diminish the returns on the incremental capital cost. Therefore, (12) stages or more can be found on the very largest commercial systems only.

The Aguayuda system is a modular vertical system than can be manufactured at relatively low cost and has up to 10 stages with a maximum energy efficiency of 9:1 and a production rate of maximum 25,000 liters / day. Larger systems are available, custom designed.