Water desalination reverse osmosis process

The successful development of asymmetric cellulose acetate membranes by Loeb and Sourirajan in the early sixties, at the University of California, Los Angeles, has been primarily responsible for the rapid development of Reverse Osmosis (RO) technology for brack sh/sea water desalination. Reverse Osmosis approaches a reversible process when the pressure barely exceeds the osmotic pressure and hence the energy costs are quite low. Theenergy requirement to purify one litre of water by RO is only O.OO3 KW as against 0,7 KV required just to supply the vaporisation energy to change the phase of one litre of water from liquid to vapour by evaporation. Thus RO has an inherent capability to convert brackish water to potable water at economic cost and thus contribute effectively to the health and prosperity of all humanity. 

One can also recognize that application of sufficient pressure (above the equilibrium osmotic pressure n) to the right-hand chamber in (7.67) must cause the solvent flow to reverse, resulting in extrusion of pure solvent from solution. This is the phenomenon of reverse osmosis, an important industrial process for water desalination. Reverse osmosis is also used for other purification processes, such as removal of H20 from ethanol beyond the azeotropic limit of distillation (Section 7.3.4). Reverse osmosis also finds numerous applications in wastewater treatment, solvent recovery, and pollution control processes. 

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). 

The economics of Reverse Osmosis Process will be highly favourable provided the desalination industry is taken up in a big way bringing down the capital investment. Water management and distribution particularly the water supply in the rural areas must be given top priority and should be under the direct control of central and federal government agencies and in this endeavour reverse osmosis has a potential... 

Reverse osmosis membranes. The exceptionally hi moisture regain observed with polybenzimidazole fibers prompted a team at Ctelanese Research Co to investigate the utility of polybenzimidazole films as semipermeable membranes for reverse osmosis processes, sudi as sea water desalination A continuous process was devised in which films were... 

Membrane research and development started in Du Pont in 1962 and culminated in the introduction of the first B-9 Permasep permeator for desalination of brackish water by reverse osmosis (RO) in 1969. The membrane in this B-9 Permasep module consisted of aramid hollow fibers. In 1969, proponents of RO technology had ambitious dreams and hopes. Today, RO is a major desalination process used worldwide to provide potable water from brackish and seawater feeds. Du Font s membrane modules for RO are sold under the trademark Permasep permeators. The RO business is a virtually autonomous profit center that resides in the Polymer Products Department. The growth and success of the Permasep products business is a direct result of Du Font s sustained research and development commitment to polyamides, a commitment that dates back to the 1930 s and the classic polymer researches of Wallace H. Carothers. Since 1969, improved and new Permasep permeators have been introduced six times, as shown in Table I. 

Eor water-treatment processes such as drinking water or potable water production, reverse osmosis (desalination), nanofiltration, and ultrafiltration are mainly used. In these processes often a microfiltration stage is implemented as the first cleaning stage for the removal of dissolved organic matter, colloids and particles from the source. 

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. 

Desalination The process of removing salts from water by reverse osmosis or distillation. 

The reverse osmosis business that Sidney Loeb pioneered is now worth many billion euros annually. Installed reverse osmosis processes produce in excess of 13.5 billion cubic metres of drinking water annually and are now the leading desalination technology on a world basis. Additionally, more than 17,000 small industrial, ship-mounted and household reverse osmosis systems are also in use. Furthermore, the closely related processes of nanofiltration and ultrafiltration are very widely used throughout the manufacmring industries, including pharmaceuticals and food production. Such membrane processes also have important medical applications. 

Types of membranes for reverse osmosis. One of the more important membranes for reverse-osmosis desalination and many other reverse-osmosis processes is the cellulose acetate membrane. The asymmetric membrane is made as a composite film in which a thin dense layer about 0.1 to 10 pm thick of extremely fine pores supported upon a much thicker (50 to 125 pm) layer of microporous sponge with little resistance to permeation. The thin, dense layer has the ability to block the passage of quite small solute molecules. In desalination the membrane rejects the salt solute and allows the solvent water to pass through. Solutes which are most effectively excluded by the cellulose acetate membrane are the salts NaCl, NaBr, CaClj, and NajSO sucrose and tetralkyl ammonium salts. The main limitations of the cellulose acetate membrane are that it can only be used mainly in aqueous solutions and that it must be used below about 60°C. 

The most successful application of the reverse osmosis process is in the production of drinking water from seawater. This process is known as seawater desalination and is currently producing millions of gallons of potable water daily in the Middle East. Fishing boats, ocean liners, and submarines also carry... 

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. 

The osmotic pressure of a solution can be counteracted by exerting additional pressure on the side of the membrane that has the more concentrated solution. In fact, ifpext is greater than IT, then the osmosis process will occur in the opposite direction. Such reverse osmosis processes have some extremely practical benefits. Perhaps the most important is the production of fresh water from seawater in desalinization plants. In the Middle East, these plants produce drinkable water from the very salty water of the gulfs and seas in the area. The process is a product of technology, but is much less energy-intensive than distillation. 

Consider reverse osmosis desalination in a spiral-wound module for a feed having negligible osmotic pressure. If the fractional water recovery, re, is such that the osmotic pressure of the concentration from the reverse osmosis process still has a negligible osmotic pressure vis-a-vis the liquid pressure, derive the result (7.2.44), i.e. 

Nakao, S. (1996). Sea water desalination process for high recovery of fresh water by reverse osmosis. Bull. Soc. Sea Water Sci. Japn. 50(6), 406-412. 

Some desalination plants combine distillation with reverse osmosis to produce both power and water. Multistage flash (MSF) processes are used to produce both power and distilled water. The combination of RO and MSF and the advantages of such a combination have been reported (111). 

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