Separation seawater desalination process

The traditional membrane separation processes (reverse osmosis, micro-, ultra- and nanofiltration, electrodialysis, perva-poration, etc.), already largely used in many different applications, are today combined with new membrane systems such as CMRs and membrane contactors. Membranes are applied not only in traditional separation processes such as seawater desalination but also in medicine, bioengineering, microelectronics, the life in the space, etc. 

Reverse osmosis is used as a method of desalting seawater, recovering wastewater from paper mill operations, pollution control, industrial water treatment, chemical separations, and food processing. This method involves application of pressure to the surface of a saline solution, thus forcing pure water to pass from the solution through a membrane that is too dense to permit passage of sodium and chlorine ions. Hollow fibers of cellulose acetate or nylon are used as membranes, since their large surface area offers more efficient separation. See dialysis membrane diffusion desalination. 

Cellulose acetate is the material for the first-generation reverse osmosis (RO) membranes. The announcement of cellulose acetate membranes for seawater desalination by Loeb and Sourirajan in 1960 triggered the applications of membrane separation processes in many industrial sectors. Cellulose acetate membranes are prepared by the dry-wet phase inversion technique. 

This book may also attract a wider range of readers, since synthetic membranes are now considered to be one of the most important tools in the areas of seawater desalination, wastewater treatment, water production, food processing, treatment of pharmaceutical products, air and water cleaning, separation of chemical and petrochemical products, drug release, and other biomedical applications. 

Chemical processes produce complex mixtures of compounds from various feedstocks. Proper operation of chemical reactors often requires that the feed contain only certain species in specified ratios. Thus, separation and purification of species from feedstocks, whether petroleum, coal, mineral ores, or biomass, must be accomplished. Similarly, a mixture leaving a reactor must be separated into purified products, byproducts, unreacied feed, and waste materials. Separation processes ate also of importance where no reaction is involved as in seawater desalination by reverse osmosis, crystallization, or evaporation in the fractionation of crude petroleum or in the drying of solids or devolalization of polymers where diffusion within a porous solid is of importance. 

The first separation example is seawater desalination. Traditionally, desahnation was done by distillation or simple evaporation/condensation [55]. Today, thermally driven desalination has been largely replaced by the membrane process reverse osmosis. In reverse osmosis an applied pressure exceeding the osmotic pressure of the salt solution causes water to permeate through a dense membrane. Hydrated salt ions are relatively large compared to water and have a lower permeability through the membrane resulting in relatively salt-free water being collected as the reverse osmosis permeate. 

Reverse osmosis. A membrane, which impedes the passage of a low-molecular-weight solute, is placed between a solute-solvent solution and a pure solvent. The solvent diffuses into the solution by osmosis. In reverse osmosis a reverse pressure difference is imposed which causes the flow of solvent to reverse as in the desalination of seawater. This process also is used to separate other low-molecular-weight solutes, such as salts, sugars, and simple acids from a solvent (usually water). This process is covered in detail in Sections 13.9 and 13.10. 

This process is widely used in a seawater desalination plant, where purified water is obtained against a high salt concentration of seawater. In the metal-making industry this purification method is used in the oil-water mixed jet-cutting tool emulsions that contain high concentration of metals. A reverse osmosis unit separates the oil from water to be reused again. 

The non-electric markets for nuclear energy potentially include seawater desalination, district heating, low temperature process heat, and high temperature heat (including a potential for hydrogen manufacture by water splitting). These markets are likely to be served by commercial entities, which are separate from electric utilities, and for which financing relies on commercial bank loan rates or usual rates of return on investor equity. 

AEMs and alkaline ionomers (anionomers) are key to the successful implementation of AMECs. Anion-exchange membranes have, for a long time, been used as separation membranes for seawater desalination, the recovery of metal ions from wastewaters, electrodialysis and bio-separation processes, for example [16-26]. These membranes may, however, not be stable or conductive enough to be applied in AMECs. AEMs used in early AMEC studies were reviewed in 2005 [1] and included polybenzimidazole (PBl) doped with KOH, epichlorohydrin polymer... 

Feng [86] and Feng et al. [87] developed novel nanofibrous membranes for seawater desalination by air-gap MD. The PVDF nanofibrous membranes were characterized by SEM, AFM, and DSC, measurement of LEPw (liquid entry pressure of water), equilibrium contact angle, and particle separation. It was found that the pore size of the PVDF nanofibrous membrane was around 1.5 pm. The equilibrium contact angle of some nanofibrous membranes was above 120°. Feng et al. [86] attempted for the first time to use ENMs for desalination by MD. PDVF nanofibrous membrane could produce potable water (NaCl concentration <280 ppm) from a saline water of NaCl concentration 6 wt% by air-gap MD (AGMD). This new approach may eventually enable the MD process to compete with conventional... 

This book was planned to commemorate the announcement of the first cellulose acetate membrane for reverse osmosis by Loeb and Sourirajan in 1960, which triggered R D activities for seawater desalination by membrane and eventually resulted in emergence of a novel industrial separation process. Membrane separation technologies that include reverse osmosis, nanofiltration, ultrafiltarion, membrane gas and vapor separation, pervaporation, membrane extraction, membrane distillation, bipolar membrane and others, touch nowadays all aspects of human life since they are applied in various branches of industries such as chemical process, petrochemical and petroleum, pharmaceutical, environmental and food processing industries. 

Reverse osmosis processes for desalination were first appHed to brackish water, which has a lower I DS concentration than seawater. Brackish water has less than 10,000 mg/L IDS seawater contains greater than 30,000 mg/L IDS. This difference in IDS translates into a substantial difference in osmotic pressure and thus the RO operating pressure required to achieve separation. The need to process feed streams containing larger amounts of dissolved soHds led to the development of RO membranes capable of operating at pressures approaching 10.3 MFa (1500 psi). Desalination plants around the world process both brackish water and seawater (15). 

If you were to place a solution and a pure solvent in the same container but separate them by a semipermeable membrane (which allows the passage of some molecules, but not all particles) you would observe that the level of the solvent side would decrease while the solution side would increase. This indicates that the solvent molecules are passing through the semipermeable membrane, a process called osmosis. Eventually the system would reach equilibrium, and the difference in levels would remain constant. The difference in the two levels is related to the osmotic pressure. In fact, one could exert a pressure on the solution side exceeding the osmotic pressure, and solvent molecules could be forced back through the semipermeable membrane into the solvent side. This process is called reverse osmosis and is the basis of the desalination of seawater for drinking purposes. These processes are shown in Figure 13.1. 

---