Desalination process

Desalination. Desalination of seawater and brackish water has been and, as of the mid-1990s, is the primary use of RO. Driven by a need for potable water in areas of the world where there is a shortage, this industry has developed. Desalination involves the reduction of the total dissolved soHds (IDS) concentration to less than 200 mg/L. RO offers several advantages over other possible desalination processes such as distillation (qv), evaporation (qv), and electro dialysis. The primary advantage of RO over the traditionally used method of distillation is the energy savings that is afforded by the lack of a phase change in RO. 

While the ambient-temperature operation of membrane processes reduces scaling, membranes are much more susceptible not only to minute amounts of scaling or even dirt, but also to the presence of certain salts and other compounds that reduce their ability to separate salt from water. To reduce corrosion, scaling, and other problems, the water to be desalted is pretreated. The pretreatment consists of filtration, and may include removal of air (deaeration), removal of CO2 (decarbonation), and selective removal of scale-forming salts (softening). It also includes the addition of chemicals that allow operation without scale deposition, or which retard scale deposition or cause the precipitation of scale which does not adhere to soHd surfaces, and that prevent foam formation during the desalination process. 

The major water desalination processes that ate currendy in use or in advanced research stages are described herein. Information on detailed modeling can be found in the Hterature cited. The major texts on water desalination written since the 1980s are those by Spiegler and Laird (47), Khan (48), which contains many practical design aspects, Lior (49) on the measurements and control aspects, Heitman (40) on pretreatment and chemistry aspects, and Spiegler and El-Sayed (50), an overview primer. Extensive data sources are provided in References 39 and 51. 

Fig. 11. Flow diagram of the vacuum-freeze (direct) vapor-compression desalination process.

A. S. Khan, Desalination Processes and Multistage Flash Distillation Practice, Elsevier, Amsterdam, the Netherlands, 1986. 

Table 5. Energy requirements of four industrial desalination processes. (Source International Atomic Energy Agency 1992.)...

Processes in which two phases react and result in the formation of a third form an important group of gas-liquid-particle processes. In the production of acetylene, a gaseous phase is formed by reaction between a liquid and a particle phase water and carbide. In the production of gas hydrates in desalination processes, a particle phase is formed by reaction between a liquid and a gaseous phase sea water and, for example, propane. In the melting of gas-hydrate or ice crystals a liquid phase is formed when gaseous and particle phases are brought in contact. 

Reverse osmosis membrane process, 27 637 Reverse osmosis membrane cleaning citric acid application, 6 647 Reverse-osmosis membranes, 75 811, 825 development of, 75 797 Reverse osmosis models, 27 638-639 Reverse osmosis permeators, 76 19 Reverse osmosis seawater desalination process, 26 85 Reverse osmosis systems blending in, 26 80-81 brackish and nanofiltration, 26 80-83 Reverse osmosis technology... 

Water desalination, 26 51-102 economic aspects of, 26 96 energy requirements for, 26 59-61 freshwater manufacture via, 26 58-61 future prospects for, 26 96-98 membrane desalination processes, 26 73-87... 

Water desalination. See also Distillation processes, Water problem Water desalination processes, 26 59, 61 Water-dominated hydrothermal resources, 12 530-533... 

The majority of the discharge from a desalination processes is concentrated brine from the membrane process, and this may contain quantities of treatment chemicals used. Treatment of water is necessary in all desalination plans for variety of reasons feed water treatment, membrane protection, membrane cleaning, permeate treatment and concentrate treatment prior to discharge. Although non-chemical treatment is possible, chemical treatment is widely practiced. 

Microfiltration (MF) and ultrafiltration (UF) membranes can be used as forms of pretreatment for nanofiltration (ISIF) or reverse osmosis (RO) desalination processes. Membrane pretreatment reduces the amount of chemicals that are required and hence reduces the environmental impact of the final discharge. MF membranes can be used to filter particles with diameters of 0.1-10 pmm and typically remove bacteria, viruses, precipitates, coagulates and large colloidal particles. UF can remove particles with diameters as small as 0.002 pm, and... 

NF has been also been studied as a potential form of pretreatment for reverse osmosis desalination processes (Hassan et al. 1998, 2000). Based on the feed water, it may be a suitable pretreatment method that allows for operation with little or even no use of antisealants. 

Cotravo, J.A. Water Desalination Processes and Associated Health and Environmental Issues. Water Cond. Purif. (January), 13-17 (2005). 

Voutchkov, N., Sommariva, C., Pankratz, T., Tonner, J. Desalination Process Technology. In Cotruvo, J.A., Voutchkov, N., Fawell, J., Payment, P., Cunlifife, D., Lattemann, S. (eds.) Desalination Technology—Health and Environmental Impacts. CRC Press, Boca Raton (2010)... 

For this reason, two pass seawater desalination process have been necessarily employed till quite recently, and the results obtained have been satisfactory to some extent with regard to water quality and practical operation. However, one pass process has advantages over two pass process for simple and compact plant, simple operation, easy maintenance and lower energy consumption. 

The purpose of the present work is to evaluate the applicability and merit of high pressure desalination process by the use of Hollosep-High Rejection Type. 

Desalination processing techniques fall into three groups ... 

Fig. 4. Schematic of reverse osmosis process used ill desalination process. (Toyobo Co., Ltd., Osaka, Japan)...

A number of other desalination processes, such as freezing, membrane distillation, and solar humidification, have been used to desalt saline waters. Based on their commercial success, these processes can be considered as minor desalination processes. 

In order to obtain potable water from sea water, one must either remove good watex from the solution or remove salt from the solution, leaving the good water behind. Most well known desalinization processes work on the principle of removing good water from solution the ion membrane process is the notable exception. It seems obvious that, since sea water is 96.5% water and only 3.5% salt, it would be preferable to remove the salt. Such processes received special attention in the work reported here. 

Recent work by Reid (13), Breton (1), and Loeb (9) shows promise that cellulose acetate may be used for such osmotic measurements. However, their objective was to study the desalinization process and not the equilibrium measurement of osmotic pressure. 

Related to the hypothetical mechanism of semipermeable membranes discussed on page 40 Siemann in my laboratory has determined water spectra in celluloseacetate - and polyimide membranes. (U. Siemann, Diplomarbeit Universitat Marburg, 1976). These membranes are useful for desalination processes. The spectra show a strong intensity of OH vibrations in the frequency region of weak bonded OH groups. 

The origin of this absorption may be water molecules with one OH group bonded to the membrane and the second OH group non-H-bonded or water molecules with H-bonds to weak acceptors of the membranes. In both cases the water transport through these membranes may be related to weak H-bonds water-membrane. In addition the spectroscopic observations show in celluloseacetate — or polyimide — membranes are less water molecules of the type of liquid water. In agreement with this observation we have found in model glass membranes with low salt rejection (about 70%) at high relative humidity water spectra not far from the spectra of liquid water. As result of these first experiments we may discuss two possible mechanism of membranes for desalination processes ... 

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