Freezing, desalination

Distillation processes, 26 61-73. See also Distillation(s) freeze-desalination, 26 71 materials and scaling issues in, 26 71-73 multi-effect distillation, 26 65-67 multistage flash evaporation, 26 61-65 vapor compression distillation, 26 67 Distillation reactors, 21 332 Distillation region diagrams (DRD), 22 302, 303, 331... 

Freeze-desalination, 26 71 Freeze dryers, 9 135-136 Freeze drying, 9 93, 116 11 543 ceramics processing, 5 656g coffee, 7 262... 

Thijssen and Spicer1 1191 has given a general review of freeze concentration as an industrial separation process and Bushnell and Eagen(63) have discussed the status of freeze desalination. The potential of freeze crystallisation in the recycling and re-use of wastewater has been reviewed by Heist 120, and the kinetics of ice crystallisation in aqueous sugar solutions and fruit juice are considered by Omran and King(121). 

Figure 8.55. Immiscible refrigerant (n-butane) freeze desalination process U.K. A.E.R.E., Harwell)...

Barduhn, A.J. (1975) The status of freeze-desalination. Chemical Engineering Progress,... 

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

Crystallisation by freezing, or freeze crystallisation, is a process in which heat is removed from a solution to form crystals of the solvent rather than of the solute. This is followed by separation of crystals from the concentrated solution, washing the crystals with near-pure solvent, and finally melting the crystals to produce virtually pure solvent. The product of freeze crystallisation can be either the melted crystals, as in water desalination, or the concentrated solution, as in the concentration of fruit juice or coffee extracts. Freeze crystallisation is applicable in principle to a variety of solvents and solutions although, because it is most commonly applied to aqueous systems, the following comments refer exclusively to the freezing of water. 

The concentrations of seawater and brackish water can vary significantly, and as such there is a difference between the concentrate produced from seawater desalination plants and brackish water desahnation plants. Seawater typically has a level of total dissolved solids (TDS) between 33,000-37,000 mg/L. The average major ion concentration of seawater is shown in Table 2.1 along with water from the Mediterranean Sea, and water from Wonthaggi off the southern coast of Australia. Seawater sahnity increases in areas where water evaporates or freezes, and it decreases due to rain, river runoff, and melting ice. The areas of greatest salinity occur and latitudes of 30° N and S where there are high evaporation rates. 

All commercial types of processes, with the exception of freezing, namely, distillation, reverse osmosis and electrodialysis, are being applied in the above units with various kinds of distillation processes being used for seawater desalting. Two of them, horizontal tube multieffect distillation and vapor compression units were developed and manufactured locally by the Israel Desalination Engineering Ltd. Recently, two small RO units with a combined capacity of approx. 100 cu. m/day were also used to desalt seawater. The main aim of these units is to test and demonstrate the feasibility of this new technology. 

How might water be desalinized by freezing What would be a major advantage and a major disadvantage of such a method ... 

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. 

Kvajic, G., "Solar Desalination by Freezing and Distillation," Proc. 2nd Intern. Conf. on Alternative Energy Sources, Vol. 3, pp. 1009-1046, Miami Beach, FL (1979). 

Desalination of seawater is one of the important applications of membrane processes. There are various ways to produce fresh water such as distillation, electrodialysis, membrane distillation, freezing, membrane bioreactor, and reverse osmosis. Among them, distillation is the most used technique, but RO is becoming more popular in the desalination industry. A flow diagram of a single-stage RO system is shown in Fig. 4. 

Cellulosic Membranes. The first asymmetric membrane for gas separation appeared in 1970 (Table II), and It was not surprising that this membrane was a modified CA membrane of the Loeb-Sourirajan type (17). Gelled CA membranes for water desalination must be stored wet In order to maintain their permeation performance. However, In gas permeation, wet, plasticized membranes tend to lose their properties with time due to plastic creep of the soft material under pressure and due to slow drying during which the microporous sublayer may collapse and thus increase the thickness of the dense skin-layer. Gantzel and Merten (17) dried CA membranes with an acetyl-content of 39.4% by quick-freezing and vacuum sublimation at... 

Both distillation and freezing involve phase changes that require considerable energy. On the other hand, desalination by reverse osmosis does not involve a phase change and is economically more desirable. Reverse osmosis uses high pressure to force water from... 

Explain why reverse osmosis is (theoretically) more desirable as a desalination method than distillation or freezing. What minimum pressure must be applied to seawater at 25°C in order for reverse osmosis to occur (Treat seawater as a 0.70 M NaCl solution.)... 

Both the lack of freezing and the inability of ions to enter the pores containing motionally restricted water can be explained by the existence of fragmented clusters such as monomors, dimers etc. Thus, the presence of these fragmented clusters prevent the necessary aggregation and co-operative expansion needed for an ice-like structure to exist, while at the same time they are less able to hydrate ions resulting in low solubilities and consequently low rejections in the desalination sense (1,2 ). This could be the microscopic mechanistic basis for the solution-diffusion model so... 

Otherwise a slow transition from a mobile to a non-mobile state occurred below 0°C. This can be explained by super-cooling and slow-freezing or an apparent phase transition effect. (Depending on the validity and applicability of the model, this difference in freezing behavior could be used as a quick diagnostic tool to determine whether a membrane is suitable for desalination or not). At low coverages (2 layers), the water behaved very similarly on both pore-size glasses. For example T1/T2, f profiles and t. were all similar. ... 

The descaling method would have particular value in desalination processes involving distillation. Some advantage in electrodialysis is also anticipated. Scale formation is not a problem in processes involving hydrate formation and freezing. 

The operation of an industrial-size gas-oil-water spray column (13 ft high, 1.3 ft diam) was reported by Barbouteau (B3). The relatively low overall heat-transfer coefficients calculated from his data, up to 1250Btu/ft /hr/°F, seems to suggest operation at relatively low holdup. Umano (Ul) studied spray column in the course of his work on direct-contact freezing units for water desalination. 

Serkiz and Perdue (1990) were the first to report the successful use of RO to isolate NOM from river water. Polyamide (PA) membranes on a polysulphone (PS) support were used. Freshwatcrs were pretreated with a cation exchange resin and, due to a low salt content, no further desalination was required. Concentrates were then freeze dried. Recoveries of about 90% were achieved. The method appears to be well suited for NOM removal with the possibility of subsequent use of XAD for separation of FA and HA from the NOM. The method can be applied to freshwatcrs containing DOC in the range of 3 to 40 mgL h Problems of the method include the presence of residual inorganics such as H2SO4 and Si(OH)4. 

Reverse osmosis uses high pressure to force water from a more concentrated solution to a less concentrated one through a semipermeable membrane. Desalination by reverse osmosis is considerably cheaper than by distillation and avoids the technical difficulties associated with freezing. 

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