The reverse osmosis process

We have indicated in the introduction to this chapter that physico-chemical processes related to the diffusion of constituents or to their solubility in the fluid inside a dense membrane should be taken into accoimt in order to model membrane separation of very small particles dispersed in a fluid. We discuss this aspect now, by describing the reverse osmosis process. 

5 See again G. Baluais, Tost-traitements a la filtration (brochure J 3 502), Techniques de Tlngenieur, volume G6nie des Procedes. 

The value of the osmotic pressure is calculated using the van t Hoff relation for osmosis  

This relation is formally identical to the ideal gas law, since Q is the concentration of particles in mohm , T the absolute temperature, and R the universal gas constant (i = 8.31 J mol K ). The Van t Hoff relation is in fact derived finm the difference of chemical potentials of the solvent on either side of the porous membrane, owing to the presence of particles in the left compartment. 

Osmotic pressure in salt water is easy to calculate. For a salt concentration of 10 g L , we obtain Posm = 8.5 bar at a temperature of 25°C. The molar mass of NaCl being 58 g (35 g + 23 g), 10 g L of salt correspond to 345 mol-m of ions. Now, it is observed that a factor 2 appears, since NaCl dissociates into Na and Cl ions (i.e. there are 172.5 mol-m of Na ions and 172.5 mol-m of Cl ions). The main finding is that the osmotic pressure is very high because, for a reasonable concentration of particles in g L , the concentration of particles in mol is large 

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The reverse osmosis process requires a pressure equal to or slightly greater than the... 

Note that a solution more concentrated than the original one also results from the reverse osmosis process. This means that the method of reverse osmosis may also be used as a method for concentrating solutions. Fruit juices and radioactive wastes, for example, have been concentrated by this method. 

After the reverse osmosis process, inorganic impurities are completely removed by anion exchanger and cation exchanger. An ultraviolet light is installed and the water is circulated to prevent microbial growth. 

Three different membrane processes, ultrafiltration, reverse osmosis, and electrodialysis are receiving increased interest in pollution-control applications as end-of-pipe treatment and for inplant recovery systems. There is no sharp distinction between ultrafiltration and reverse osmosis. In the former, the separation is based primarily on the size of the solute molecule which, depending upon the particular membrane porosity, can range from about 2 to 10,000 millimicrons. In the reverse-osmosis process, the size of the solute molecule is not the sole basis for the degree of removal, since other characteristics of the... 

Microfiltration—A reverse osmosis process that removes particles larger than those removed by ultrafiltration and operating at the lowest operating range of the reverse osmosis processes in the neighborhood of 70 kPag. 

The equilibrium of sulfide in water, the percentages of H2S, HS, and species, is dependent on the pH. Figure 1 shows the distribution of each species at various pH. At a pH of approx 5.7, the sulfide species in water would be near 100% H2S and at approx pH 7, 50% of the sulfide species in water would be H2S and the other 50% would be HS species. The H2S species are volatile as a result, the aeration process effectively removes it from the water. Therefore, the removal efficiency of sulfide depends on pH. As the pH increases, aeration becomes less effective because there are fewer sulfides in the form of H2S, which is readily removed by aeration. This process is utilized by both municipalities and chemical industries. In water treatment, the process is called degasification, and is effectively used to remove both H2S and carbon dioxide from well water and product water from the reverse osmosis process. 

To meet the operating requirements of the reverse osmosis process, a pumping system Is commonly used to pressurize and circulate feed fluids at sufficient velocities to maintain the desired turbulence and to provide pressurized make-up fluid to replace withdrawn permeate and concentrate fluids. 

Referring to the equations above, "the reversible isothermic work for the reverse osmosis process can be described by... 

While the above equations are helpful in describing the reverse osmosis process, the water and salt transport coefficients seldom are used to describe membrane performance. Most manufacturers test reverse osmosis membrane with standard solutions as described below. 

Many natural waters are saturated in calcium carbonate which is highly rejected by the membrane. Consequently, it is concentrated in the feed/reject stream during the reverse osmosis process and it will precipitate on the membrane decreasing the flux and rejection. Lowering the pH of the feedwater to between 4 and 6 converts some of the carbonate or bicarbonate ions to carbon dioxide and this prevents carbonate precipitation. 

Some feedwaters contain compounds in addition to calcium carbonate that may become saturated in the feed/reject stream and when that stream is concentrated in the reverse osmosis process, these compounds will precipitate on the membrane with a resultant loss of membrane performance. Some of the more sparingly soluble compounds of concern are ... 

The reverse osmosis process is relatively simple and instrumentation requirements are minimal. Following is a list of the minimum recommended parameters to be measured in a reverse osmosis system ... 

The above applications were specific examples that were chosen to demonstrate the versatility of the reverse osmosis process. The following list enumerates a wide variety of applications for which the reverse osmosis process may be considered ... 

Abstract The processes of lactic acid production include two key stages, which are (a) fermentation and (h) product recovery. In this study, fiee cell of Bifidobacterium longum was used to produce lactic acid from cheese whey. The produced lactic acid was then separated and purified from the fermentation broth using combination of nanofiltration and reverse osmosis membranes. Nanofiltration membrane with a molecular weight cutofif of 100-400 Da was used to separate lactic acid from lactose and cells in the cheese whey fermentation broth in the first step. The obtained permeate from the above nanofiltration is mainly composed of lactic acid and water, which was then concentrated with a reverse osmosis membrane in the second step. Among the tested nanofiltration membranes, HL membrane from GE Osmonics has the highest lactose retention (97 1%). In the reverse osmosis process, the ADF membrane could retain 100% of lactic acid to obtain permeate with water only. The effect of membrane and pressure on permeate flux and retention of lactose/lactic acid was also reported in this paper. 

The compartments indicated in Figure 5.57a are a schematic representation of reverse osmosis process. In practice, the reverse osmosis process is conducted in a tubular configuration system (Figure 5.57b). Raw waste water flows under high pressure (greater than osmotic pressure) through an inner tube made... 

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). 

Poly(benzimidazoles) are speciality polymers that have found only a few civilian applications outside the military area and in space research. PBI fibers have been used for thermally stable protective clothing and as precursors in the production of graphite fibers. Hollow fibers and films of PBI are used to purify sea and brackish water according to the reverse osmosis process. 

Reverse osmosis is the transfer of solvent (water) through a semi-permeable membrane under the effect of a pressure gradient. As a flux is limited by the mass transfer in the membrane, the pressure gradient applied can be important. It is thus frequent to work with pressures of about 70 or 100 bar. The reverse osmosis process is usually used to obtain pure water from sea water, brackish water, or waste water. 

Figure VIH - 30. Schematical representation of the reverse osmosis process.

Reverse Osmosis Reverse osmosis (RO) is a filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurised side of the membrane and the pure solvent is allowed to pass to the other side. To be selective, this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely. The reverse osmosis process is used for desalting of brackish water, in the food industry, in maple syrup production and so forth. This technology is not frequently used primarily due to high membrane replacement cost. 

The reverse osmosis process, also known as hyperfiltration , is based on the passage of solvent molecules through a dense membrane from a concentrated solution to a dilute one. As this process is opposed by osmotic pressure, the pressure drop across the membrane must be higher to overcome it. Figure 11.8 shows how simple the process is in principle. The solution is pumped over a membrane held on a permeable support and it is split into the solvent (permeate) and concentrate streams. 

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... 

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