High-pressure seawater reverse osmosis

Goosen M.F.A., Sablani S., Dal Cin M., Wilf M., Jackson D., Al-Belushki R., and AlMaskri S., Effect of high temperature and pressure on permeation properties of composite polyamide seawater reverse osmosis and brackish water nanofiltration membranes. Journal of Membrane Science (Submitted 2008). 

Both the brackish and seawater reverse osmosis product water costs are based on 1982 costs and they are indicative of specific plants in an assumed location in the southern United States. The cost of energy in the seawater system assumes that the reject from the first stage high pressure reverse osmosis system is sent to an energy recovery system which reduces the overall energy requirements for the total system by 31%. 

Memhra.nes. Liquid separation via membranes, ie, reverse osmosis (qv), is used in production of pure water from seawater. The chief limit to broader use of reverse osmosis is the high pressure required as the concentration of reject rises. 

The pressure to be used for reverse osmosis depends on the salinity of the feedwater, the type of membrane, and the desired product purity. It ranges from about 1.5 MPa for low feed concentrations or high flux membranes, through 2.5—4 MPa for brackish waters, and to 6—8.4 MPa for seawater desalination. In desalination of brackish or sea water, typical product water fluxes through spiral-wound membranes are about 600—800 kg/m /d at a recovery ratio RR of 15% and an average salt rejection of 99.5%, where... 

Reverse osmosis can be used to purify water, because the liquid passing through the semipermeable membrane is pure solvent. A water purifier that uses reverse osmosis requires semipermeable membranes that do not rapture under the high pressures required for reverse osmosis. Recall that seawater has an osmotic pressure of nearly 28 atm and that red blood cells rupture at 7 atm. Nevertheless, membranes have been developed that make it feasible to purify water using this technique. Reverse osmosis currently supplies pure drinking water to individual households as well as entire municipalities. 

The Disc Tube system is a patented, ex situ process for the treatment of aqueous solutions ranging from seawater to leachate. The system uses high-pressure reverse osmosis through a semipermeable membrane to separate pure water from contaminated liquids. 

Reverse osmosis is a process in which freshwater is obtained from saltwater by forcing water from a region of low freshwater concentration to a region of high freshwater concentration. This is opposite of the natural process, and hence the name reverse osmosis. In a typical reverse osmosis process, a pressure of approximately 30 atmospheres is required to force freshwater to move from seawater across a semiperme-able membrane (Figure 11.5). 

Reverse osmosis performs a separation without a phase change. Thus, the energy requirements are low. Typical energy consumption is 6 to 7 kWh/m2 of product water in seawater desalination. Reverse osmosis, of course, is not only used in desalination, but also for producing high-pressure boiler feedwater, bacteria-free water, and ultrapure water for rinsing electronic components—because of its properties for rejecting colloidal matter, particle and bacteria. 

Since the discovery by Cadotte and his co-workers that high-flux, high-rejection reverse osmosis membranes can be made by interfacial polymerization [7,9,10], this method has become the new industry standard. Interfacial composite membranes have significantly higher salt rejections and fluxes than cellulose acetate membranes. The first membranes made by Cadotte had salt rejections in tests with 3.5 % sodium chloride solutions (synthetic seawater) of greater than 99 % and fluxes of 18 gal/ft2 day at a pressure of 1500 psi. The membranes could also be operated at temperatures above 35 °C, the temperature ceiling for Loeb-Sourirajan cellulose acetate membranes. Today s interfacial composite membranes are significantly better. Typical membranes, tested with 3.5 % sodium chloride solutions,... 

Reverse osmosis membranes can be divided into subclasses according to their solute/water selectivity and operating pressure regimes. Figure 30 shows a number of commercial membranes developed for seawater and brackish desalination, and for nanofiltration. These include cellulose ester and polyamide asymmetric membranes available since the 1960s, and high-performance composite membranes developed in the 1970s. Collectively, they make it possible to produce potable water from virtually all saline water sources. 

Due to the added resistance of the membrane, the applied pressures required to achieve reverse osmosis are significantly higher than the osmotic pressure. For example, for 1,500 ppm TDS brackish water, RO operating pressures can range from about 150 psi to 400 psi. For seawater at 35,000 ppm TDS, RO operating pressures as high as 1,500 psi may be required. 

On the other hand, pressure is applied in reverse osmosis to drive the solvent (water) out of the high-concentration side into the low-concentration side this facilitates de-watering insoluble species for their removal. This process produces high-quality water and concentrated refuse. It separates and removes dissolved solids, organics, pyrogens, colloidal matter, viruses, and bacteria from water in the particle range 10 4—10—2 pm. Reverse osmosis can remove up to 95%-99% of the total dissolved solids (TDS) and 99% of all bacteria. It is used for the ob-tention of drinking water from seawater and for the production of ultra pure water in various industries. 

An interesting alternative development is that of forward osmosis. Whereas in reverse osmosis a high pressure is required to oppose the natural tendency of freshwater to move across such a membrane via osmosis to dilute the seawater, in forward osmosis the system takes advantage of this natural tendency. Here, salt water sits on one side of the membrane, but the freshwater on the opposite side is transformed into a high-concentration solution by adding NH3 and CO2. Water naturally flows from the salt water to what is now the draw solution, which can have a solute concentration as high as 10 times that of the salt water. There is no need for an external pressure. The diluted draw solution is then heated to evaporate off the CO2 and NH3 for reuse, leaving behind freshwater. (See Patel-Predd, 2006). 

Oceans hold about 97% of the earth s water supply, but their high salt content makes them unsafe for drinking or agriculture. Salt can be removed by placing the seawater in contact with a semipermeable membrane, then subjecting it to pressures greater than 60 atmospheres. Under these conditions, reverse osmosis occurs, pushing the water molecules out of the seawater into a reservoir of pure water. 

Since the electrical resistance of the effiuent and parasitic currents are minimal at high level of impurities, specihc interest in electrically assisted membrane processes could increase due to more strict laws and legislation around effluents. The depletion of freshwater resources and the necessity to process brackish or seawater to produce potable water could promote the use of electrically assisted membrane processes in the future. Electrodialysis will have to compete with pressure-driven membrane processes such as reverse osmosis. The growing awareness of the unique cleaning ability of electrically ionized water (EIW) [47], a byproduct of electrodialysis, may be a factor to consider in the choice between ED and RO systems. NMR relaxation measurements were used to determine the water cluster size of electrically ionized water EIW. It is known that the water cluster size of EIW is signihcantly smaller than that of tap water. The smaller water cluster size is believed to enhance the penetration and extractive properties of EIW. Recently, EIW has been produced and used in several cleaning processes [47] in industry. 

Membrane processes of the reverse osmosis (hyperfiltration) or electrodialysis types are used, but usually for smaller scale facilities (Fig. 5.4). Reverse osmosis units use high pressures of brackish water or seawater charging on one side of a semipermeable membrane, sufficient to exceed the osmotic... 

The properties of FT-30 membranes have been reviewed in several publications, including reverse osmosis performance under seawater and brackish water test conditions.60"62 In commercially produced spiral-wound elements, the FT-30 membrane typically gives 99.1 to 99.3% salt rejection at 24 gfd flux in seawater desalination at 800 psi and 25°C. In brackish water applications, FT-30 spiral elements can be operated at system pressures of as low as 225 psi while producing water at 22 to 24 gfd. Similar flux levels are possible with the TFC-202 and LP-300 membranes, as mentioned earlier. But it is notable that those membranes achieve such high fluxes through use of extremely thin surface barrier layers about only one-tenth the thickness of the FT-30 barrier layer. 

If a solution in contact with pure solvent across a semipermeable membrane is subjected to an external pressure larger than its osmotic pressure, reverse osmosis occurs. The pressure will cause a net flow of solvent from the solution to the solvent, as shown in Fig. 11.20. In reverse osmosis, the semipermeable membrane acts as a "molecular filter" to remove solute particles. This fact is applicable to the desalination (removal of dissolved salts) of seawater, which is highly hypertonic to body fluids and thus is not drinkable. 

Seawater or brackish water can be purified by reverse osmosis. To maximise the flow of water through a polymer membrane, the polymer must have a high water permeability, yet a low permeability for the salts. To maximise efficiency, the membrane area must be large and its thickness as small as possible consistent with a lack of pinholes. A high pressure is applied to the salt water side of the membrane. Because it is thin, a cellulose triacetate membrane is supported on a porous cellulose nitrate-cellulose acetate support structure to resist the pressure. To make the unit compact the composite membrane is spirally wound on to an inner cylinder, and the edges glued together. When a pressure of 70 bar is applied to the seawater side, NaCl rejection levels in excess of 99.7% can be achieved. 

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