Seawater reverse osmosis process

Geraldes, V., N. E. Pereira, and M. Norberta de Pinho, Simulation and Optimization of Medium-Sized Seawater Reverse Osmosis Processes with Spiral-Wound Modules, Ind. Engr. Chem. Research, 44,1897 (2005). 

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). 

Fig. 2.1 Increase in concentration factor and decrease in concentrate volume with increasing recovery rate. The shaded regions represent typical recovery ranges for typical seawater reverse osmosis (SWRO) and brackish water reverse osmosis (BWRO) processes...

Co-location of a power plant and a seawater reverse osmosis desalination plant allows for the cooling water from a neighbouring power plant to be blended with the waste from a desalination plant before discharge (Voutchkov 2004). In such a process, seawater is used as the cooling water for the condensers in a power plant. This water is then used as both the feed for the desahnation process, and for blending to dilute the concentrate from the desalination plant. 

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). 

Lorain, O., B. Hersant, F. Persin, A. Grasmick, N. Brunard, and J. M. Espenan. 2007. Ultrafiltration membrane pre-treatment benefits for reverse osmosis process in seawater desalting. Quantification in terms of capital investment cost and operating cost reduction. Desalination 203 277-285. 

Generally, the salt rejections observed for these membranes in seawater reverse osmosis tests did not exceed 80 percent. This process was applied at Albany International to form composite membranes on hollow polysulfone fibers (25). Salt rejections on the hollow fiber membranes were above 98 percent at an average flux of about 1.5 gfd (2.5 L/sq m/hr) in a 12 000-hour test using 30 000 ppm seawater at 1000 psi. In other 5000-hour tests using 3500 ppm brackish water at 400 psi with addition of 100 ppm chlorine at pH 8 flux and salt rejection remained constant at 1 gfd and 98 percent respectively (Figures 8 and 12 in Reference 25 a). 

Kunisada Y, Murayama Y, and Hirai M, The development of seawater desalination by reverse osmosis process in Japan, Desalination 1977, 22, 243-252. 

Determine the operating pressure to produce 1 ms of freshwater from 2 ms seawater (3.5% NaCl wt) by the reverse-osmosis process shown in Figure irt-16. Assume that the water in the brine that exits the unit is at equilibrium with the clean water in the filtrate and neglect any nonideal effects. The feed is at ambient conditions (25 "C, 1 bar). 

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 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. 

This explains why reverse osmosis processes employ very high-pressure levels. An exercise at the end of this chapter examines various aspects of the dimensioning of a seawater desalination process. 

Concentration of Seawater by ED. In terms of membrane area, concentration of seawater is the second largest use. Warm seawater is concentrated by ED to 18 to 20% dissolved soHds using membranes with monovalent-ion-selective skins. The EDR process is not used. The osmotic pressure difference between about 19% NaCl solution and partially depleted seawater is about 20,000 kPa (200 atm) at 25°C, which is well beyond the range of reverse osmosis. Salt is produced from the brine by evaporation and crystallisa tion at seven plants in Japan and one each in South Korea, Taiwan, and Kuwait. A second plant is soon to be built in South Korea. None of the plants are justified on economic grounds compared to imported solar or mined salt. 

Industrial Wastes. Closely related to seawater concentration is the simultaneous concentration of industrial effluents and recycle of recovered water (see Wastes, industrial). These appHcations are expected to increase as environmental restrictions increase. Examples are the concentration of blowdown from cooling towers in power plants concentration of reverse osmosis blowdown and the processing of metal treatment wastes (11) (see... 

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... 

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. 

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