Desalination, of water

Low viscosity cellulose propionate butyrate esters containing 3—5% butyryl, 40—50% propionyl, and 2—3% hydroxyl groups have excellent compatibihty with oil-modified alkyd resins (qv) and are used in wood furniture coatings (155). Acetate butyrate esters have been used in such varied apphcations as hot-melt adhesive formulations (156), electrostatically spray-coated powders for fusible, non-cratering coatings on metal surfaces (157—159), contact lenses (qv) with improved oxygen permeabiUty and excellent wear characteristics (160—162), and as reverse-osmosis membranes for desalination of water (163). 

By the time the next overview of electrical properties of polymers was published (Blythe 1979), besides a detailed treatment of dielectric properties it included a chapter on conduction, both ionic and electronic. To take ionic conduction first, ion-exchange membranes as separation tools for electrolytes go back a long way historically, to the beginning of the twentieth century a polymeric membrane semipermeable to ions was first used in 1950 for the desalination of water (Jusa and McRae 1950). This kind of membrane is surveyed in detail by Strathmann (1994). Much more recently, highly developed polymeric membranes began to be used as electrolytes for experimental rechargeable batteries and, with particular success, for fuel cells. This important use is further discussed in Chapter 11. 

The conditions utilized in the above development of minimum energy are not sufficient to describe electrodialysis. In addition to the desalination of water, salt is moved from a saline feed to a more concentrated compartment. That free-energy change must be added to the free energy given in Eq. (20-107), which describes the movement... 

Applications of reverse osmosis are normally restricted to below 50°C. Reverse osmosis is now widely applied to the desalination of water to produce potable water. Other applications include ... 

Crystallisation processes employing DCC have been used successfully in the de-waxing of lubricating oils 63, the desalination of water(64), and the production of inorganic salts from aqueous solution. 

The new concept of using mixed-matrix membranes with commercially attractive thin-film composite geometry for desalination of water has been demonstrated by Jeong and coworkers [83]. 

In the case of desalination of water by reverse osmosis illustrated in Figure 4.3(a), the salt concentration cio adjacent to the membrane surface is higher than the bulk solution concentration c, because reverse osmosis membranes preferentially permeate water and retain salt. Water and salt are brought toward the membrane surface by the flow of solution through the membrane J,.1 Water and a little salt permeate the membrane, but most of the salt is rejected by the membrane and retained at the membrane surface. Salt accumulates at the membrane surface until a sufficient gradient has formed to allow the salt to diffuse to the bulk solution. Steady state is then reached. 

In the United States, electrodialysis was developed primarily for desalination of water, with Ionics being the industry leader. In Japan, Asahi Glass, Asahi Chemical (a different company), and Tokuyama Soda developed the process to concentrate seawater [8], This application of electrodialysis is confined to Japan, which has no domestic salt sources. Electrodialysis membranes concentrate the salt in seawater to about 18-20% solids, after which the brine is further concentrated by evaporation and the salt recovered by crystallization. 

Various phenomena which we now associate with adsorption were known in antiquity. The adsorbent properties of such materials as clay, sand and wood charcoal were utilized by the ancient Egyptians, Greeks and Romans (Robens, 1994). These applications were wide-ranging and included the desalination of water, the clarification of fat and oil and the treatment of many diseases. 

Later, our system proved to work successfully also for desalination of water, but the development through the pharmaceutical to the food and dairy industries and pulp and paper industry did give a great deal of experience which has been extremely valuable for our design of water desalination plants as constructed today. 

In some applications, the interaction of the polymer with a specific "solvent" and/or with certain molecules carried by that solvent is not a detrimental event, but an essential aspect of the operation of the polymer. Reverse osmosis membranes and swollen hydrogels used in applications such as the desalination of water, kidney dialysis, soft contact lenses and surgical implants [17] are among such polymers. 

Membrane Separations. Separation processes using polymeric membranes (30) have become important techniques because of their simplicity and low consumption of energy in comparison to alternatives such as distillation. Membranes for ultrafiltration are porous, and no diffusive transport actually occurs through the polymer itself. However, for separation at the molecular level, diffusion through the polymer provides a possible mechanism for selective passage of the desired small molecule. Reverse osmosis for desalination of water can occur by this mechanism, and large commercial processes using this technique are now in operation. 

Neutralization dialysis Separation of electrolyte and nonelectrolyte, desalination of water, etc. 

Reverse osmosis (3) Liquid Pressure gradient to overcome osmotic pressure Membrane Desalination of water, Vol. 14, p. 347... 

Electrodialysis (11) Liquid Electric field plus charged membranes Membrane and electric field Desalination of water, Vol. 7, p. 857... 

Water-soluble and water-swelling polyampholytes are used in a wide number of applications including sewage treatment, flocculation, enhanced oil recovery etc. The desalination of water by cross-linked polyampholytes can be regulated by changing the temperature. Such polyampholytes are called thermoregenera-ble resins (TRR). To perform the thermoregeneration the next equilibrium should take place in dependence of temperature ... 

The dialyzer used for desalination of water by electrodialysis (Figure 5.55) is an electric cell divided into a series of sub-cells that are separated by alternate polyanion and polycation membranes. While the ions move under the influence of the applied electric potential, the complimentary permselectivities of the ion-exchange membranes insure that salt is removed from alternate sub-cells and concentrated in the others. Desalination dialyzers used in practice have up to 100 such sub-cells. 

Over time, various applications were demonstrated on laboratory or pilot plant scale, including the fractionation of gases, the concentration of aqueous solutions, the desalination of water, cool energy storage, and the storage and transport of natural gas. All of these await future developments of commercially viable technologies. 

The following processes were all discussed in Chapter 18, Chemistry of the Environment Estimate whether the entropy of the system increases or decreases during each process (a) photodissociation of 02(g), (b) formation of ozone from oxygen molecules and oxygen atoms, (c) diffusion of CFCs into the stratosphere, (d) desalination of water by reverse osmosis. 

Inorganic and organic substances, which can exchange their own ions for others without change in stability. They are employed for the softening (desalination) of water... 

The desalination of water by freezing is well known 178,179). Similarly, the use of zone refining as a means of purifying solids is well established 180,181). Very little, however, has been done with these methods to preconcentrate inorganic solutes from water for analytical purposes. The subject has, however, been reviewed from a theoretical and practical viewpoint 182). 

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