Multistage Flash Distillation

Multi-stage flash distillation is a process that uses distillation through many (multi-stage) chambers. The most important thing about MSF is that as the process enters the new phase the pressure lowers down. For every successive stage, the plant operates at lower pressure.

In this process Brine Heater is used to heat the seawater. Brine heater is basically a container in which heating is done by condensing steam on a bank of tubes carrying sea water through the brine heater. Once heated the water is transferred to another container which is called as a stage. The pressure in the stage is lower than the pressure of the brine heater. The introduction of water in lower pressure chamber results in the boiling of water and it's sudden evaporation 'flashing'. In every successive stage this 'flashing' of a portion of the feed continues. This is because the pressure at each stage is lower than the preceding stage. Flashing produces vapor that is condensed on heat exchanger tubing which run through each stage. The cooler feed water helps in the cooling of the tubes. Since only a small fraction of this water is converted into steam therefore the remaining water is sent in series of stages each stage has lower pressure than the previous stage.



At every stage, the vapor pressure is controlled in order to enable the heated brine to enter each chamber at the proper temperature and pressure that is each lower than the preceding stage thereby causing instantaneous boiling and evaporation.

If plant is heated at the high temperature then it will enhance the thermal efficiency but operation at high temperature can also increase the potential for detrimental scale formation and accelerated corrosion of metal surfaces.


Working of MSF Plant
Nearly 85% of the world's total desalinated water is produced by multi-stage flash distillation plants. MSF distillation plants are installed along with power plants. The wasted heat of the power plant is utilized to to heat the seawater. This lower the cost of energy to a greater extent.

In MSF plant, the quantity of flashing brine required is quite large. More then half or two third of the brine coming out of last stage is mixed with the incoming feed water, and then recirculated through the heat recovery sections of the brine heater. They are later flashed again through all of the consequent stages. This type of facility is referred as a "brine recycle" plant. Though this method can reduce the cost of operation yet it is a fact that it increases the salinity of the brine, increases the danger of corrosion and scaling in the plant. It is therefore necessary to maintain a proper brine density in the system.