What is Multi Stage Flash Distillation MSF

The Multi Stage Flash distillation MSF evaporator vessels is also referred to as flash stages or effects. What happens is that the high-salinity source water is heated to a temperature of 90 to 115°C (194 to 239°F) in a vessel (the heating section in the figure below) to create water vapor. The pressure in the first stage is maintained slightly below the saturation vapor pressure of the water. So when the high-pressure vapor created in the heating section enters into the first stage, its pressure is reduced to a level at which the vapor “flashes” into steam. Steam (waste heat) for the heating section is provided by the power plant co-located with the desalination plant. Each flash stage (effect) has a condenser to turn the steam into distillate. The condensers are equipped with heat exchange tubes, which are cooled by the source water that is fed to the condensers.

schematic multi stage flash distillation process

Entrainment separators (mist eliminators or demister pads) remove the high-salinity mist from the low-salinity rising steam. This steam condenses into pure water (distillate) on the heat exchange tubes and is collected in distillate trays. This is from where it is conveyed to a product water tank. Distillate flows from stage to stage and is collected at the last stage. The concentrate (brine) is generated in each stage and after collection at the last stage some of it typically is recycled to the source water stream in order to reduce the total volume of source water that must be collected by the intake for desalination. The recirculated brine flowing through the interior of the condenser tubes also removes the latent heat of condensation.

As a result, the recirculated brine is also preheated close to maximum operating temperature. Thereby recovering the energy of the condensing vapor and reducing the overall heating needs of the source water. This “brine recycle” feature has been adopted in practically all of the most recent MSF facility designs and allows significant improvement of the overall cost competitiveness of MSF installations. Each flash stage typically produces approximately 1 percent of the total volume of the desalination plant’s condensate. Since a typical MSF unit has 19 to 28 effects, the total MSF plant recovery (i.e., the volume of distillate expressed as a percentage of the total volume of processed source water) is typically 19 to 28 percent. For comparison, Reverse Osmosis seawater desalination SWRO plants have a recovery of 40% to 45%. The latest Multi Stage Flash Distillation technology has 45-stage units—i.e., can operate at 45% recovery. This feature allows it to compete with Reverse Osmosis systems in terms of recovery.

Historically, MSF was the first commercially available thermal desalination technology. It was applied to produce potable water on a large-scale, which explains its popularity. MSF plants represent over 80% of thermally desalinated water today. The gained output ratio GOR for MSF systems is typically between 2 and 8; the latest MSF technology has a GOR of 7 to 9. The pumping power required for the operation of the MSF systems is 2.0 to 3.5 kWh/m3 (7.6 to 13.3 kWh/1000 gal) of product water.

Reference: “Desalination Engineering” by Nikolay Voutchkov