Water desalination
Methods
Distillation Multi-stage flash distillation (MSF) Multiple-effect evaporator (MED|ME) Vapor-compression (VC) Ion exchange Membrane processes Electrodialysis reversal (EDR) Reverse osmosis (RO) Nanofiltration (NF) Membrane distillation (MD) Freezing Geothermal desalination Solar humidification (HDH, MEH) Methane hydrate crystallization High grade water recycling RF Induced Hyperthermia Seawater greenhouse

Multi-stage flash distillation (MSF) is a water desalination process that distills sea water by flashing a portion of the water into steam in multiple stages of what are essentially regenerative heat exchangers. Reverse osmosis plants are the most common type, but multi-stage flash distillation plants produce over 85 percent of all desalinated water in the world.

Contents 1 Principle 2 See also 3 References 4 External links

[edit] Principle

First, the seawater is heated in a container known as a brine heater. This is usually achieved by condensing steam on a bank of tubes carrying sea water through the brine heater. Heated water is passed to another container known as a "stage", where the surrounding pressure is lower than that in the brine heater. It is the sudden introduction of this water into a lower pressure "stage" that causes it to boil so rapidly as to flash into steam. As a rule, only a small percentage of this water is converted into steam. Consequently, it is normally the case that the remaining water will be sent through a series of additional stages, each possessing a lower ambient pressure than the previous "stage." As steam is generated, it is condensed on tubes of heat exchangers that run through each stage.

Such plants can operate at 23-27kWh/m³ of distilled water.^[1]

Because the colder salt water entering the process counterflows with the saline waste water/distilled water, relatively little heat energy leaves in the outflow- most of the heat is picked up by the colder saline water flowing into the process and the energy is recycled.

In addition, MSF distillation plants, especially large ones, are often paired with power plants in a cogeneration configuration. Waste heat from the power plant is used to heat the seawater, providing cooling for the power plant at the same time. This reduces the energy needed by one-half to two-thirds, which drastically alters the economics of the plant, since energy is by far the largest operating cost of MSF plants. Reverse osmosis, MSF distillation's main competitor, requires more pretreatment of the seawater and more maintenance, as well as energy in the form of work (electricity, mechanical power) as opposed to cheaper low-grade waste heat.^[2][3]

[edit] See also

• Multiple-effect evaporation
• Reverse osmosis
• Regenerative heat exchanger

[edit] References

[edit] External links

• International Desalination Association
• Encyclopedia of Desalination and Water Resources