Seawater osmotic pressure

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). 

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. 

The freezing point, temperature of maximum density, osmotic pressure and specific heat for seawater of various salinities are given in Table 21.23. 

Reverse osmosis, (a) When the external pressure (P) is less than the osmotic pressure (71-) (P < it), normal osmosis occurs, (b) When the external pressure exceeds the osmotic pressure, water flows in the opposite direction, producing reverse osmosis. Reverse osmosis can be used to obtain fresh water from seawater. 

Plastic membrane This is done by the use of a water permeable plastic membrane held deep enough under the sea so that the hydrostatic pressure is greater than the osmotic pressure of the seawater. The water distills out of the solution through the membrane and is pumped to the surface. Large areas of the membranes, mechanically supported to withstand the very high pressures are essential to make the process perform rapidly for the most economical production. 

The high osmotic pressure of seawater, 27.6 atm, makes it clear that osmotic pressures can be quite large. Thus, an osmotic pressure of 6.9 atm is a reasonable value for a solution such as blood. 

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. 

C12-0100. Saltwater fish have blood that is isotonic with seawater, which freezes at -1.96 ° C. What is the osmotic pressure of fish blood at 15 °C ... 

Assuming that each ion acts independently of all the others and that seawater has a density of 1.026 g/mL, calculate the freezing point and osmotic pressure of seawater. The actual freezing point of seawater is - 1.96 ° C. What conclusion can you reach about your assumptions Take this into account and recalculate the osmotic pressure of seawater. 

All eukaryote cells are faced with differences in intracellular solute composition when compared with the external environment. Many eukaryotes live in seawater, and have cells which are either bathed in seawater directly, or have an extracellular body fluid which is broadly similar to seawater [3]. Osmoregulation and body fluid composition in animals has been extensively reviewed (e.g. [3,15-21]), and reveals that many marine invertebrates have body fluids that are iso-osmotic with seawater, but may regulate some electrolytes (e.g. SO2-) at lower levels than seawater. Most vertebrates have a body fluid osmotic pressure (about 320mOsmkg 1), which is about one-third of that in seawater (lOOOmOsmkg ), and also regulate some electrolytes in body fluids at... 

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. 

In reverse osmosis, a pressure equal to the osmotic pressure of seawater is applied to obtain freshwater from seawater. 

FIGURE 11.16 A schematic for the desalination of seawater by reverse osmosis. By applying a pressure on the seawater that is greater than osmotic pressure, water is forced through the osmotic membrane from the seawater side to the pure water side. 

Colligative properties have many practical uses, including the melting of snow by salt, the desalination of seawater by reverse osmosis, the separation and purification of volatile liquids by fractional distillation, and the determination of molecular mass by osmotic pressure measurement. 

Figure 2.9 Flux and rejection data for a model seawater solution (3.5 % sodium chloride) in a good quality reverse osmosis membrane (FilmTec Corp. FT 30 membrane) as a function of pressure [10]. The salt flux, in accordance with Equation (2.44), is essentially constant and independent of pressure. The water flux, in accordance with Equation (2.43), increases with pressure, and, at zero flux, meets the pressure axis at the osmotic pressure of seawater 350 psi...

Figure 5.25 Effect of water recovery on the seawater feed osmotic pressure and net driving pressure of a plant operating at 1000 psi...

Application of RO for water production is now a well-accepted and economical process even for higher concentration seawater with osmotic pressures of over 300 psi. Malta, for example, has evolved an economical reliable application of this technology to produce 60% of its potable water supply (Lamendolar and Tua, 1995). 

Brackish waters contain between 0.05 and 1 wt % TDS. Their lower osmotic pressures allow reverse osmosis operation between 15 and 30 bar. Less expensive pressure equipment and energy consumption translate to more favorable water production economics than those for seawater desalination. 

Osmotic pressure (typically represented by jt (pi)) is a function of the concentration of dissolved solids. It ranges from 0.6 to 1.1 psi for every 100 ppm total dissolved solids (TDS). For example, brackish water at 1,500 ppm TDS would have an osmotic pressure of about 15 psi. Seawater, at 35,000 ppm TDS, would have an osmotic pressure of about 350 psi. 

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. 

Figure 4.1 Flux and rejection data for a seawater FilmTec FT-30 membranes operating on 35,000 ppm (350 psi osmotic pressure) sodium chloride solution.2...

A process called reverse osmosis is used to remove salts from seawater to make drinking water for human consumption. If a pressure greater than the osmotic pressure is applied to the solution side of an apparatus such as shown in Figure 15.6, water is forced from the solution (the seawater) to the pure solvent (water) side. This process is used industrially for water purification. 

Osmotic pressure is another property due to dissolved substances. The presence of solute particles lowers the ability of solvent molecules to pass through a semipermeable membrane. Osmotic pressure is very important in biological systems, and an application of the theory behind osmotic pressure allows for the purification of seawater. The osmotic pressure of a solution, IT, is proportional to the molarity (the number of moles per liter) ... 

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