Middle East Desalination

Huntington Beach Proposal

Sources Cited

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Multi-effect Distillation

Saline water is evaporated on one side of a tube wall while steam is condensed on the other using heat of condensation.

History

Multi-effect Distillation, or MED, was used primarily by industry to evaporate juice to get sugar and to evaporate water to get salt as commodities.  There has also been use of this distillation method for drinking water desalination.  Earlier configurations were plagued by scaling problems due to the high temperatures involved, and for this reason have been largely replace by Multi-stage Flash.  However, newer processes developed during the 1980's are able to operate at lower temperatures to minimize scaling among other problems.

Locations that use MED include the Caribbean, India, Canary Islands, and the United Arab Emirates.  Typically these plants can handle from 0.5 to 5 million gallons per day.  Their size is dependent on the number of effects used which usually number from 8 to 16, and efficiency of the plant increases as the number of effects increases in part to an increase in heat transfer area. 

Mechanism

Steam that condenses inside the effects is what ends up as drinking water, and a vacuum pump controls ambient pressure in the effects.  Heat exchangers come in different types, and the most common consist of horizontal tubes with a falling film.  The final condenser is itself a heat exchanger in which incoming seawater is preheated by the condensate energy while the condensate is cooled by the seawater.  This circulation of condensate increases the efficiency as well since energy needed to heat the seawater is acquired from within the system.

Operating temperatures run around 70 degrees Celsius.

image source:http://www.iwra.siu.edu/win/win2000/win03-00/semiat.pdf

Benefits

1) a low operating cost when waste heat is used for the distillation process

2) the quality of the feed water is not as important as for a reverse osmosis system

3) the multi-effect flash system has a high GOR

4) the multi-effect flash system can utilize hot water instead of steam for the
distillation process. Hot water temperature is less affected by changes in the power
 plant generator load than steam production

Cons

1) high operating costs when waste heat is not available for the distillation process

2) the multi-effect flash system operates at high temperatures that increase corrosion and scale formation

National Science Foundation: Office of Polar Programs. "Sea water desalination system, McMurdo" 1993.

http://www.nsf.gov/pubs/stis1993/opp93104/opp93104.txt

Sources

Buros, O.K. "The ABC's of Desalting" Second Ed. Topsfield, MA  http://www.idadesal.org/ABCs1.pdf

National Science Foundation: Office of Polar Programs. "Sea water desalination system, McMurdo" 1993.

http://www.nsf.gov/pubs/stis1993/opp93104/opp93104.txt

UNEP - International Environmental Technology Centre. United Nations Environment Programme. "Source Book of Alternative Technologies for Freshwater Augmentation in Latin America and the Caribbean" 1997.