Dessalement de l'eau de mer- Potentiel chimique- osmose inverse (Mai 2024)
Le dessalement, également appelé dessalage, élimination des sels dissous de l'eau de mer et, dans certains cas, des eaux saumâtres (légèrement salées) des mers intérieures, des eaux souterraines hautement minéralisées (par exemple, des saumures géothermiques) et des eaux usées municipales. Ce processus rend ces eaux autrement inutilisables propres à la consommation humaine, l'irrigation, les applications industrielles et diverses autres fins. La technologie de dessalement existante nécessite une quantité substantielle d'énergie, généralement sous forme de combustibles fossiles, et donc le processus est coûteux. Pour cette raison, il n'est généralement utilisé que lorsque les sources d'eau douce ne sont pas économiquement disponibles. De plus, la quantité d'émissions de gaz à effet de serre et d'eaux usées de saumure générées par les usines de dessalement pose des défis environnementaux importants.
Sciences de la terre: Désalinisation, marémotrice et minéraux de la mer
Pendant des siècles, source de nourriture et de sel commun, la mer devient de plus en plus une source d'eau, de produits chimiques et d'énergie. En 1967, Key West,
Le dessalage de l'eau de mer est une notion ancienne. Aristote a décrit une méthode d'évaporation utilisée par les marins grecs du IVe siècle av. Un écrivain arabe du 8e siècle a produit un traité sur la distillation. Au 19e siècle, le développement de la navigation à vapeur a créé une demande d'eau non corrodée pour les chaudières et le premier brevet pour un procédé de dessalement a été accordé en Angleterre en 1869. La même année, la première usine de distillation d'eau a été construite par le gouvernement britannique à Aden, pour approvisionner les navires s'arrêtant au port de la mer Rouge. Le premier grand alambic à fournir de l'eau à des fins commerciales a été construit en 1930 à Aruba, près du Venezuela.En 2019, environ 18000 usines de dessalement produisent un total de plus de 95 millions de mètres cubes (plus de 3,4 milliards de pieds cubes) d'eau potable par jour. étaient en service dans le monde entier.
Desalination processes
Desalination methods can utilize either thermal processes (involving heat transfer and a phase change) or membrane processes (using thin sheets of synthetic semipermeable materials to separate water from dissolved salt). Multistage flash distillation is a thermal process for desalting relatively large quantities of seawater. Based on the fact that the boiling temperature of water is lowered as air pressure drops, this process is carried out in a series of closed tanks (stages) set at progressively lower pressures. When preheated seawater enters the first stage, some of it rapidly boils (flashes), forming vapour that is condensed into fresh water on heat-exchange tubes. Fresh water is collected in trays as the remaining seawater flows into the next stage, where it also flashes, and the process is continued. One of the largest of these systems, located in Al-Jubayl, Saudi Arabia, can produce more than 750 million litres (200 million gallons) of desalted water per day.
In small communities where salt water and intense sunlight are both abundant, a simple thermal process called solar humidification can be used. The heat of the Sun partially vaporizes salt water under a transparent cover. On the underside of the cover, the vapour condenses and flows into a collecting trough. The principal difficulty in this process is that large land areas are required, and energy is needed for pumping the water. Another thermal process makes use of the fact that, when salt water is frozen, the ice crystals contain no salt. In practice, however, objectionable amounts of salt water remain trapped between the crystals, and the amount of fresh water needed to wash the salt water away is comparable to the amount of fresh water produced by melting the crystals.
Membrane processes for desalting include reverse osmosis and electrodialysis. Of the two, reverse osmosis is the more widely used, particularly for desalting brackish waters from inland seas. The salt content of brackish inland water, though undesirable, is considerably below that of seawater. Electrodialysis uses electrical potential to drive the positive and negative ions of dissolved salts through separate semipermeable synthetic membrane filters. This process leaves fresh water between the filters. In reverse osmosis salt water is forced against the membranes under high pressure; fresh water passes through while the concentrated mineral salts remain behind. To conserve space, the membranes are packaged in multiple layers in a collection of long tubes. One of the largest reverse-osmosis desalination plants now in operation is located in Sorek, Israel, and can produce some 627,000 cubic metres (22 million cubic feet) of desalted water per day.
Global production
In many areas of the world, particularly in densely populated arid regions, desalted water is the main source of municipal water supplies. Desalination is used in more than 120 countries, and about half of all desalted water is produced in the Middle East and North Africa. By 2019 the largest producers of desalinated water were Saudi Arabia, the United Arab Emirates, and Kuwait. The United States is another major producer, accounting for roughly 13 percent of the total output (mostly in Florida, Texas, and California). The majority of all desalination plants are reverse-osmosis systems, with multistage flash distillation being the second-ranking process.
In general, a population usually can afford to pay about 7–10 times as much for water for domestic purposes as it does for agricultural water. Large-scale desalination facilities promise to lower the cost of desalted water at the desalination sites to a level that most industries and some agricultural enterprises can afford. In the future it can be expected that the ocean will become an increasingly important source of fresh water. If production and transportation costs can be lowered sufficiently, it may be possible to produce fresh water to irrigate large areas that border the oceans in many parts of the world.
