Multi-stage flash distillation


Multi-stage flash distillation
Water desalination
Methods

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 countercurrent heat exchangers. Multi-stage flash distillation plants produce 64% of all desalinated water in the world [1], although a different type of desalinators, reverse osmosis plants, are more numerous.

Contents

Principle

Schematic of a multi-stage flash desalinator
A - Steam in
B - Seawater in
C - Potable water out
D - Waste out
E - Steam out
F - Heat exchange
G - Condensation collection
H - Brine heater

The plant has a series of spaces called stages, each containing a heat exchanger and a condensate collector. The sequence has a cold end and a hot end while intermediate stages have intermediate temperatures. The stages have different pressures corresponding to the boiling points of water at the stage temperatures. After the hot end there is a container called the brine heater.

When the plant is operating in steady state, feed water at the cold inlet temperature flows, or is pumped, through the heat exchangers in the stages and warms up. When it reaches the brine heater it already has nearly the maximum temperature. In the heater, an amount of additional heat is added. After the heater, the water flows through valves back into the stages which have ever lower pressure and temperature. As it flows back through the stages the water is now called brine, to distinguish it from the inlet water. In each stage, as the brine enters, its temperature is above the boiling point at the pressure of the stage, and a small fraction of the brine water boils ("flashes") to steam thereby reducing the temperature until an equilibrium is reached. The resulting steam is a little hotter than the feed water in the heat exchanger. The steam cools and condenses against the heat exchanger tubes, thereby heating the feed water as described earlier.

The total evaporation in all the stages is up to approx. 15% of the water flowing through the system, depending on the range of temperatures used. With increasing temperature there are growing difficulties of scale formation and corrosion. 120 °C appears to be a maximum, although scale avoidance may require temperatures below 70 °C.

The feed water carries away the latent heat of the condensed steam, maintaining the low temperature of the stage. The pressure in the chamber remains constant as equal amounts of steam is formed when new warm brine enters the stage and steam is removed as it condenses on the tubes of the heat exchanger. The equilibrium is stable, because if at some point more vapor forms, the pressure increases and that reduces evaporation and increases condensation.

In the final stage the brine and the condensate has a temperature near the inlet temperature. Then the brine and condensate are pumped out from the low pressure in the stage to the ambient pressure. The brine and condensate still carry a small amount of heat that is lost from the system when they are discharged. The heat that was added in the heater makes up for this loss.

The heat added in the brine heater usually comes in the form of hot steam from an industrial process co-located with the desalination plant. The steam is allowed to condense against tubes carrying the brine (similar to the stages).

The energy that makes possible the evaporation is all present in the brine as it leaves the heater. The reason for letting the evaporation happen in multiple stages rather than a single stage at the lowest pressure and temperature, is that in a single stage, the feed water would only warm to an intermediate temperature between the inlet temperature and the heater, while much of the steam would not condense and the stage would not maintain the lowest pressure and temperature.

Such plants can operate at 23-27kWh/m3 (appr. 90MJ/m3) of distilled water.[2]

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 toward the heater 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.[3][4]

See also

References

External links


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Flash evaporation — Flash (or partial) evaporation is the partial vaporization that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit… …   Wikipedia

  • Distillation (disambiguation) — Distillation is a method of separating mixtures based on differences in their volatilities in a boiling liquid mixture. Distillation may also refer to: Distillation (Erin McKeown album), 2000 Distillation (Wishbone Ash album), 1997 Distillation… …   Wikipedia

  • Distillation — Distiller and Distillery redirect here. For other uses, see Distiller (disambiguation) and Distillery (disambiguation). For other uses, see Distillation (disambiguation). Laboratory display of distillation: 1: A heating device 2: Still pot 3:… …   Wikipedia

  • Multiple-effect distillation — Water desalination Methods Distillation Multi stage flash distillation (MSF) Multiple effect distillation (MED|ME) Vapor compression (VC) Ion exchange Membrane processes Electrodialysis reversal (EDR) Reverse osmosis (RO) Nanofiltration (NF)… …   Wikipedia

  • Continuous distillation — Image 1: Typical industrial distillation towers Image 2: A crude oil vacuum distillation column as used in oil refineries …   Wikipedia

  • Desalination — This article is about removing salt from water. For soil desalination, see Soil salinity control. Water desalination Methods Distillation Multi stage flash distillation (MSF) Multiple effect distillation (MED|ME) Vapor compression (VC) Ion… …   Wikipedia

  • Multiple-effect humidification — Water desalination Methods Distillation Multi stage flash distillation (MSF) Multiple effect distillation (MED|ME) Vapor compression (VC) Ion exchange Membrane processes Electrodialysis reversal (EDR) Reverse osmosis (RO) Nanofiltration (NF)… …   Wikipedia

  • Shoaiba power and desalination plant — The Shoaiba power and desalination plant is a oil fired CCGT power and desalination complex in Saudi Arabia on the coast of Red Sea, about convert|120|km|mi south of Jeddah. It is one of the world s largest fossil fuel power plants, and the world …   Wikipedia

  • Seawater — in the Strait of Malacca Seawater is water from a sea or ocean. On average, seawater in the world s oceans has a salinity of about 3.5% (35 g/L, or 599 mM). This means that every kilogram (roughly one litre by volume) of seawater has… …   Wikipedia

  • Water supply and sanitation in Saudi Arabia — differs from most other countries in three significant aspects. First, it relies almost exclusively on two sources that are absent in most other countries: desalination and fossil water. Second, given the substantial oil wealth of the country,… …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.