Sea water desalination is the production of drinking water and process water for industrial or power plants of sea water ( salt water ) by reducing the salt content . Desalination can be based on various processes that remove salts and minerals from the water. In some cases, there are usable by-products such as table salt .
Areas of application
In the Middle East , the energy-intensive extraction of drinking and process water using fossil fuels such as heavy oil or natural gas is widespread. In the oil-rich Gulf States, seawater desalination is the main source of drinking water production, but it also serves to obtain operating and cooling water for larger caloric power plants located on the coast . One example is the Shoaiba power plant , which as of 2014 is the largest power plant in Saudi Arabia . The water required for the cooling systems is obtained by gas- or oil-fired desalination systems in the immediate vicinity of the power plant; combined gas and steam power plants with an attached MSF desalination system are also used. The abbreviation MSF stands for English Multi Stage Flash Evaporation in the importance of multi-stage flash evaporation. As of 2014, the world's largest seawater desalination plant is the Jabal Ali power plant and seawater desalination plant in the United Arab Emirates .
In individual cases, the waste heat from nuclear power plants can also be used for seawater desalination. One example is the decommissioned Aqtau nuclear power plant , which generated 150 MW of electrical energy and 200 MW of process heat to desalinate seawater from the Caspian Sea . The waste heat from the nuclear reactor is used for seawater desalination on aircraft carriers .
On the Canary Islands and the German island of Helgoland , drinking water is obtained using the reverse osmosis process. Smaller areas of application are also the desalination of seawater on ships and submarines .
In all cases, the desalinated water is not suitable for immediate use as drinking water. In addition, such low-salt water is corrosive for ferrous materials, as no lime-rust protective layer can be formed. Subsequent addition of calcium hydrogen carbonate therefore increases the carbonate hardness content in the water again. Calcium hydrogen carbonate is produced by a reaction of calcium hydroxide (milk of lime) with carbon dioxide (CO 2 ). The CO 2 required for this is often obtained by burning natural gas .
The following section lists the usual processes in the order of their economic importance. The process of multi-stage flash evaporation (MSF) is most widespread and is used on a large industrial scale. In addition to these processes, processes of solar seawater desalination are also used to a lesser extent .
Multi-stage flash evaporation
This is a thermal process with the abbreviation "MSF" ( English Multi Stage Flash Evaporation ). It is the most commonly used method for seawater desalination. The precursor was the multi-effect distillation.
In this process, the supplied seawater is heated to a temperature of 115 ° C with the waste heat from a caloric power plant , in rare cases also from a nuclear power plant . The salt water heated in the so-called brine heater evaporates in downstream expansion stages under vacuum , the water vapor is deposited as condensate within these stages on pipes filled with cooling liquid and is drawn off as salt-free water. The water, which is increasingly enriched with salt as a result of the evaporation process, is also called brine and is cooled in a downstream heat exchanger to the condensation temperature (≈40 ° C) of the steam of the fresh water supplied. It then serves as a cooling liquid in the pipelines. The pipes themselves are continuously cleaned of crystallizing salt with sponge rubber balls . Finally, fresh salt water is fed to the brine and the mixture is heated up again by the waste heat from the gas turbine. The entire process is therefore a closed cycle . The excess of the salt that is concentrated in the cycle is returned to the sea.
Large-scale plants, such as the Jabal Ali power plant and seawater desalination plant , the world's largest seawater desalination plant, desalinate 2.135 million cubic meters of seawater every day. Usually, up to 500,000 cubic meters of drinking water are extracted from the seawater every day with the process . Similar amounts are also produced by the oil power plants in the region .
The energy consumption is 23-27 kWh / m 3 (approx. 90 MJ / m 3 ).
In reverse osmosis , the solution (seawater) is pressed through a semipermeable membrane made of polyamide , PTFE or sulfonated copolymers with a pore diameter of 0.5 to 5 nm to overcome the osmotic pressure under high pressure . This acts like a filter and only lets certain ions and molecules through. A separation of the original solution is thus obtained. The membrane filter can hold back salts , bacteria , viruses , an oversupply of lime and toxins such as heavy metals.
The osmotic pressure increases with increasing salt concentration, so the process would come to a standstill at some point. To counteract this, the concentrate is discharged. Since the crystallization of the salt or the minerals ( precipitation ) in the membranes has to be prevented, the use of reverse osmosis only makes sense up to a certain maximum concentration of the reflux. Depending on the salt concentration, due to the high pressure, even in optimal systems, an energy consumption of between 2 and 4 kWh per cubic meter of drinking water must be expected.
The membranes of a reverse osmosis system are not maintenance-free. The formation of deposits, caused by mineral deposits ( scaling ), biological substances ( biofouling ) or colloidal particles, reduces the permeation of water molecules through the membranes. To counteract this, the membranes have to be rinsed with chemical cleaners. Anti-scaling agents such as polyphosphoric acid and polymaleic acid as well as biocides and chlorine against bacterial deposits are common. These cleaning agents or rinsing water are not environmentally friendly and must be separated or treated before being returned to the concentrate ( sea ).
The drinking water treatment systems can be equipped with additional pre-filters depending on the type of water pollution. Coarse material can be separated up to a particle size of 20 micrometers. An additional activated carbon filter separates organic substances such as pesticides. UV irradiation can also be used downstream, which represents an additional level of safety against germs.
A plant that works according to this principle is the Mossel Bay seawater desalination plant in South Africa.
In the membrane distillation process , a microporous membrane is used that only allows water vapor to pass through, but retains liquid water. On one side of the membrane there is warm salt water and on the other side a colder surface. The countercurrent operation of the system ensures that there is a temperature difference over the entire length of the membrane. The resulting difference in water vapor partial pressure causes water molecules to move from the warm to the cold side of the membrane.
In the following section, various test methods for desalination are listed, some of which are also used in smaller plants.
Evaporation hoses made of plastic
As part of a European CRAFT project, the French research center CEA / GRETh has developed a seawater desalination plant in which the metal components have largely been replaced by polymers . This has the advantage that plastics corrode much less and are therefore more resistant than metals. By using plastic, the process can run under normal conditions at 100 ° C and 1 bar. The device achieves a drinking water production capacity of 100 l / h. Since the water is heated to 100 ° C, it is largely sterile and only contains small amounts of salt.
When sea water cools down, ice crystals form that are free of salts. However, the technical difficulties essentially consist in separating the ice crystals from the mother liquor . The ice crystals have to be washed from the mother liquor. In turn, there is a considerable need for fresh water, which has made this process fail in practice.
Salt water is fed into four pools. In pool 1, the salt concentration is increased (e.g. through evaporation, solar). The resulting concentrated brine in basin 1 is connected via selective polystyrene membranes (which block Na + ions to basin 2 and 3 Cl - ions to basin ), which results in an excess of Na + or Cl - ions in these basins . These two basins are connected to the fourth basin by membranes. The ions diffuse from this fourth basin to balance the ions in basins 2 and 3. The water in basin 4 is thus NaCl-free. If other salts have to be removed, other ion filters should be used. A pilot plant with environmental grants was built in Canada. The Siemens group operates a pilot plant in Singapore.
The advantage is the low energy consumption, provided that the evaporation in pool 1 is caused by the sun. The mineral content with the exception of sodium and chloride is retained, so that no other minerals have to be added to use drinking water. Additional ion filters are required for other purposes.
Bio fuel cell
Research on bio-fuel cells for desalination of weakly saline waters is carried out at the University of Queensland , Tsinghua University and the Oak Ridge National Laboratory , USA. Practical use is also being considered for brackish water.
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