Desalination of sea water

Desalination of sea water, or purification to eliminate dangerous ionic contaminants from industrial waste water involves important technological, scientific and financial risks. Most of them are related to the development of cheaper smart membranes that can mimic biological membranes. 

As discussed by Pletcher 24, electrodialysis is an electrically driven membrane separation process. The main use of electrodialysis is in the production of drinking water by the desalination of sea-water or brackish water. Another large-scale application is in the production of sodium chloride for table salt, the principal method in Japan, with production exceeding 106 tonne per annum. 

In addition to the use of existing hydrates, it has also been suggested that clathrate hydrates could find applications in storage and separation technology. Bardhun showed in 1962 the viable application of hydrates in desalination of sea water, while hydrates have also been investigated as a method for the storage and transport of methane without the use of high-pressure containers. 

Reverse osmosis Liquid Non-porous membrane with pressure gradient Desalination of sea water... 

To stop the osmosis occurring, the pressure P, in Figure 8.3, can be applied to the left-hand side. This pressure will be equal to the osmotic pressure exerted by the solution in the opposite direction. If the external applied pressure, P, is greater than the osmotic pressure then reverse osmosis occurs and molecules can be forced to pass from the stronger to the weaker solution. In this process, the semi-permeable membrane acts as a molecular filter to remove the solute particles. In some areas of the world this process is used for desalination of sea water, i.e. getting rid of salts from water. It is also used in emergency life raft survival kits to enable drinking water to be made from sea water. 

Chloride ions easily replace water molecules in the butane clathrate hydrate, and counter-cations can occupy the voids in the lattice. This is one of the reasons why the hydrate crystallization method proved unsuccessful for desalination of sea water. 

A big number of publications were appeared last years in research of WSP-metal ion interactions [111]. Interactions of metal ions with WSP are mainly due to electrostatic forces and the formation of coordinating bonds. Other weak interactions may appear such as trapping metal ions in the bulk of the polymer phase. The author stated that WSP are very perspective for selective separation and concentration (enrichment) of metal ions from the treated diluted solutions (wastewaters, desalination of sea water). 

Refer to Figure 35-1, p. 414. Suppose you apply a pressure on the left side to force the solvent back across the membrane to the other side. This is called reverse osmosis and, as a separation technique, has many commercial applications. The most notable is the desalinization of sea water. The removal of dissolved salts in hard water to make it soft water is done extensively. Many chemistry laboratories use reverse osmosis to obtain "distilled water". It is used to treat industrial waste water and to concentrate fruit juices. 

Steric criteria are also valuable to explain the formation of liquid hydrate clathrates (Fig. 3, 16.2.2.1). They appear for molecules having dimensions between the free diameters of the largest voids of types I and II structures, d, 2 and dn,2- The situation is the same for double liquid hydrates, but here a type I structure is theoretically possible in some cases, e.g., CHjBr, COS and CH3I. For molecules with dimensions >690 pm, no hydrates are formed. This selectivity in encaging certain molecules but not others has been used for fractionation of natural gas by clathration and for desalination of sea water "... 

Very little work has been reported on the use of osmotic membranes for desalinization of sea water or similar concentrations of strong electrolytes. The difficulties are associated with the problems of finding a membrane adequately semipermeable to these electrolytes and capable of operation for considerable periods at the operating pressures required. Breton (I) has reviewed the available literature. 

A scheme for an n-stage expansion evaporation unit with usage of the vapor heat is presented in Fig. 7-19. It can be used, for example, for the desalination of sea water. 

As far as the heat utilisation system is concerned, several alternatives were analysed during the design development, since one of the fundamental design criteria of the MARS reactor is the possibility of using the nuclear power produced, not only for the production of electric power, but also for other industrial purposes such as the production of steam for industrial uses (desalination of sea water district heating). 

The pool-type reactor RUTA-20 was developed as a heat source for district heating in small settlements and towns. Apart from heating purposes, the reactor can be used as a power source for summer air conditioning in residential and industrial complexes. It is also possible to use the reactor as a power source for desalination of sea water. Thermal power of a single unit is 20 MW. 

The minimum amount of energy necessary for the desalination of sea water can be obtained by simple thermodynamic calculations. When 1 mol of solvent (in this case water) passes through the membrane, the minimum work done when the process is carried out reversibly is ... 

Desalination of (sea)water is an illustrative example of a separation problem for which competitive separation processes, based on different separation principles and consunung different amounts of energy, can be used. 

Another area of government-funded academic research is the desalinization of sea water. Fresh water is becoming scarcer, and multiple studies are seeking to find ways to produce large quantities of fresh water. 

Although known for over two decades, reverse osmosis has only been accepted commercially in the last ten years, mainly due to the development in membrane technology. It is now used on a large scale in portable water treatment, for desalination of sea water, for concentration of sulphite-spent liquor and generally as a pre-concentration step before evaporation or thermal processing. There is a wide energy margin in favour of reverse osmosis as compared with evaporation but the final concentrations economically achievable are limited. The types of equipment used are the same as in ultra-filtration that follows. 

Reverse osmosis p/np 10 -lO pm Ap (<200 bar) 1/1 Desalination of sea water waste water treatment concentration of fruit juices... 

Table 3.3.15 shows that thermal desalination of sea water or brackish water by multistage flash distillation is more energy intensive than membrane desalination, but can better deal with more saline water and delivers even higher permeate quality, although reverse osmosis usually fulfills the requirements of drinking water (Table 3.3.16). 

RO membranes are dense semi-permeable membranes mainly used for desalination of sea water [38], Contrary to MF and UF membranes, RO membranes have no distinct pores. As a result, high pressures are applied to increase the permeability of the membranes [16]. The properties of the various types of membranes are summarized in Table 9.2. 

The process of reverse osmosis is used to purify water because it removes many contaminants at relatively low cost. The osmosis process can be reversed by applying a pressure greater than the osmotic pressure to an aqueous solution. This causes the water, but not the dissolved solutes, to flow from the solution through the semipermeable membrane. The largest municipal desalination plant in the United States is in Sarasota, Florida. It treats salt water by reverse osmosis to produce more than 12 million gallons of fresh water per day. Other applications include the desalination of sea water on ocean-going vessels and wastewater purification. 

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