Multi-stage flash distillation

Multi-stage flash distillation is a 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.

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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.

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 from 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.^[1][2]

See also

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

References